''^'''' I  Hill  mil  mil  mil  nil  nil 
HX00020389 


A    MANUAL 


DENTAL    ANATOMY 


HUMAN    AND    COMPARATIVE 


CHARLES    S.   TOMES,   M.A.,   F.Pv.S. 


WITH    191     ILLUSTRATIONS 


SECOND    EDITION 


PHILADELPHIA 
PRESLEY    BLAKISTON 

1882 


LONDON : 
BRADBURY.    AGXEW,    &    CO.      PRTNTERS,    WHITEFRIARS 


PREFACE. 

In  introtlucing  a  Second  Eilition  of  tins  Manual  but 
few  "words  are  necessary.  It  has  been  entirely  revised, 
some  portions,  notably  the  chapter  on  the  dental  tissues, 
have  been  in  great  part  rewritten,  and  many  new  illus- 
trations have  been  added  here  and  elsewhere  in  the 
book. 

My  indebtedness  to  Professor  Owen's  "  Odonto- 
graphy," to  my  fiithcr's  "Dental  Surgery,"'  and  to 
his  "  Lectures  on  Dental  Physiology  and  Surgery,"  to 
Professor  Flower's  "  licctures  on  the  Teeth  "  (published 
in  the  B)-itis]i  Medical  Journal,  1871^,  to  Kolliker's 
and  to  Strieker's  Histologies,  I  will  again  acknow- 
ledge, since  it  is  both  impossible  and  undesirable  to 
encumber  a  student's  book  with  references  to  au- 
thorities for  every  statement.  And  if  I  have  any- 
where failed  to  give  due  acknowledgment  to  another 
whose  writings  I  may  have  made  use  of,  I  crave  forgive- 
ness for  my  omission.  I  gladly  embrace  this  opportimity 


PREFACE 


of  expressing  my  thanks  to  Professor  Hollaender  for  the 
lionoro"  he  has  done  to  my  book  in  rendering  it  into 
German,  and  to  Dr.  Cruet  for  translating  it  into 
French  ;  and  at  the  same  time  I  gratefully  acknow- 
ledge the  kindness  of  many  of  my  friends  who  have  sent 
me  corrections  which  they  have  noted  as  being  required 
in  the  text  of  the  first  edition. 


CHARLES   S.  TOMES. 


r,  Cavendish  SyuARH 
December,  1881. 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/manualofdentalaOOtome 


A   MANUAL 


DENTAL     ANATOMY 


TABLE    OF    CONTENTS. 


CHAPTER  I. 

PACE 

The  Nature  of  Teeth — Description  oi''  thr  Teeth  of  Man       .       1 


CHAPTER  II. 
The  Maxillary  Boxes,  axd  Associated  Parts        .         .         .     .     23 


CHAPTER  III. 

The    Dental    Tissues  :     Ei^amel,    Dentine,    Cemextum,    Tooth 

Pulp,  &c 40 


CHAPTER  IV. 

The    Development    of   the    Teeth — in    Fish — in   Reptiles — m 

Mammals — Calcification  op  the  Dental  Tissues     .         .     .  113 


CHAPTER  V. 

The  Development  op  the  Jaws  and  the  Eruption  and  Attach- 
ment op  the  Teeth 17( 


CHAPTER  VI. 
The  Teeth  of  Fishes 214 


CONTENTS. 


CnAPTER  VII. 

PACK 

Thk  Tkkth  of  Batkaciua  and  Kki'Tilia 23!> 


CHAPTER  YIII. 

TiiK   Tketh   op   Mam>ials — Introductory   Kkmaiiks — Homologies 

o?  THIS  Teeth — Milk  Dentition       .         .  ...         .     .   264 


CHAPTER  IX. 

TuK  Tketh  of  Monoire.mata,  Edentata,   and  Cetacea        .         .   304 

CHAPTER   X. 
TuE  Tkktii  of  Ungclata 314 


CHAPTER  XL 

TuK  Tekth  of  Sirenia,  Hvracoidea,  Proboscidea,  and  Kodeniia  344 


CHAPTER  XII. 
TuK  Teeth  of  Caknivora 374 


CHAPTER  XIII. 
ToK  Tekth  of  Insectivora,  Ohikoi'tera,  and  Primates      .         .  394 


CHAPTER  XIV. 
The  Teeth  of  Marsuitalia    .         .         .         , 


MANUAL   OF   DENTAL  ANATOMY 

HUMAN     AND     COMPAKATIYE. 


CHAPTER    I. 

THE   TEETH    OF   MAX. 


The  range  of  the  subject  of  Dental  Anatomy  tui'ns  upon 
the  meaning  -which  is  attached  to  the  word  "  Tooth  ; "  but, 
although  this  chapter  might  most  appropriately  open  with 
a  definition  of  this  word,  it  is  very  much  easier  to  explain 
what  is  ordinarily  undei-stood  by  it,  than  to  frame  any 
single  sentence  which  shall  fulfil  the  requirements  of  logical 
definition.  Most  vertebrate  and  a  great  many  invertebrate 
animals  have  certain  hard  masses  in  or  near  to  the  orifice 
of  the  alimentary  canal,  i.e.,  the  mouth  ;  by  these  hard 
masses,  sometimes  of  bony  and  sometimes  of  horny  nature, 
various  offices  in  connection  with  the  prehension  or  com- 
minution of  food  are  performed,  and  to  them  the  term 
''  teeth  "  is  applied.  In  many  animals  teeth  have  come  to 
be  used  for  other  purposes,  such  as  for  sexual  wai-fare ;  but 
it  can  hardly  be  doubted  that  teeth  have  primarily  to  do 
with  the  nourishment  of  their  possessor. 

The  subject  of  the  homologies  of  the  teeth  cannot  be  fully 
entered  upon  until  the  details  of  their  development  have 


2  A    MANUAL    OF   DENTAL    ANATOMY. 

been  mastered ;  still  a  few  words  may  even  at  the  outset  be 
devoted  to  the  elucidation  of  their  real  nature. 

The  mucous  membrane  which  lines  the  alimentary  canal 
is  continuous  with — is,  indeed,  a  part  of — the  external  skin, 
with  which  it  blends  at  the  lips.  Now  if  a  young  dog-fish, 
just  about  to  be  hatched,  be  examined,  it  will  be  found  that 
it  has  no  distinct  under  lip,  but  that  its  skin  turns  in  over 
its  rounded  jaw  without  interruption.  The  skin  outside 
carries  spines  (placoid  scales), (')  and  these  spines  are  con- 
tinued over  that  part  of  it  which  enters  the  mouth  and 
bends  over  the  jaws ;  only  they  are  a  little  larger  in  this 
latter  position.  If  the  growth  of  the  dog-fish  be  followed, 
these  spines  of  the  skin  which  cover  the  jaws  become  deve- 
loped to  a  far  greater  size  than  those  outside,  and  the  identity 
and  continuity  of  the  two  become  to  some  extent  masked. 
No  one  can  doubt,  whether  from  the  comparison  of  adult 
forms  or  from  a  study  of  the  development  of  the  parts, 
that  the  teeth  of  the  shark  correspond  to  the  teeth  of  other 
fish,  and  these  again  to  those  of  reptiles  and  mammals  ;  it 
may  be  clearly  demonstrated  that  the  teeth  of  the  shark 
are  nothing  more  than  highly  developed  spines  of  the  skin, 
and  thei-efore  we  infer  that  all  teeth  bear  a  similar  relation 
to  the  skin.  This  is  what  is  meant  when  teeth  are  called 
"  dermal  appendages,"  and  are  said  to  be  perfectly  distinct 
from  the  internal  bony  skeleton  of  the  animal ;  the  teeth  of 
the  shark  (and  of  many  other  creatures)  remain  imbedded  in 
tough  mucous  membrane,  and  never  acquire  any  connection 
with  the  bone.  Indeed,  all  teeth  alike  are  developed  from  a 
part  of  the  mucous  membrane,  and  any  connection  which 
they  may  ultimately  get  with  the  bone  is  a  secondary 
matter.    As  it  has  been  well  expressed  by  Dr.  Harrison  Allen 

(^)  "The  placoid  scale  lias  tlie  structure  of  dentine  ;  is  covered  by 
€namel,  and  is  continued  at  its  base  into  a  plate  formed  of  osseous  tissue." 
Oegenbaur's  Comparative  Anatomy,  translated  by  F.  Jeifery  Bell,  p.  424. 


THE    TEETH    OF   MAN. 


('Anatomy  of  the  Facial  Region'),  "if  the  hairs  of  the 
scalp  were  to  be  inserted  into  the  skull,  or  of  the  moustache 
into  the  upper  jaw,  we  should  express  great  astonishment, 
yet  such  an  extreme  proposition  is  no  more  remarkable  than 
what  is  seen  to  take  place  in  the  jaws,"  again  "  the  feathers  of 
certain  birds  making  impressions  on  the  radius,  the  whalebone 
pendent  from  the  roof  of  the  mouth,  are  examples  of  this 
same  association  of  tegumentary  appendages  with  the 
bones." 

In  their  simpler  forms,  then,  teeth  are  met  with  as  ver}' 
numerovis  spines,  differing  but  little  from  the  spines  of  the 
skin  except  in  size,  and  still  less  from  one  another.  In 
many  fish  the  teeth,  though  more  specialised,  are  scattered 
over  almost  every  one  of  the  numerous  bones  which  form 
part  of  the  walls  of  the  mouth  and  pharynx ;  in  reptiles 
they  are  much  more  limited  in  position,  and  in  mammals 
are  absolutely  confined  to  the  intermaxillaiy,  maxillaiy, 
and  mandibular  (lower  maxillary)  bones.  In  fish  and  rep- 
tiles it  is  the  exception  for  the  teeth  in  different  parts  of 
the  mouth  to  differ  markedly  from  each  other;  in  mam- 
mals it  is  the  rule. 

Teeth  owe  their  hardness  to  an  impregnation  with  salts 
of  lime  ;  the  organic  matrix  may  be  of  albuminoid  charac- 
ter, in  which  case  the  tooth  is  of  horny  consistence,  and  is 
spoken  of  as  "  cornified  ;  "  or  the  matrix  may  be,  like  that 
of  bone,  gelatigenous,  in  which  case  the  tooth  is  more  richly 
impregnated  with  salts,  and  is  spoken  of  as  "  calcified." 

The  great  mass  of  a  calcified  tooth  is  usually  made  up  of 
"dentine,"  which  gives  to  it  its  characteristic  form,  and 
often  practically  constitutes  the  whole  tooth  :  to  this  may 
or  may  not  be  added  enamel  and  cementum. 

Without  further  prelude  we  may  pass  to  a  description  of 
the  human  teeth,  this  comrse  appearing  to  me,  after  some 
little  consideration,  to  afford  to  the  student  the  most  ad- 

B  2 


A    MANUAL    OF  DENTAL    ANATOMY. 


vantageous  introduction  to  the  subject,  as  he  must 
sarily  ah-eady  possess  some  knowledge  of  their  forms,  while 
to  the  matters  alluded  to  in  the  preceding  pages  more  full 
reference  will  be  made  hereafter. 

In  the  human  subject  no  tooth  rises  above  the  level  of 
its  fellows,  and  the  teeth  are  arranged  in  close  contact,  with 
no  interspaces  between  them.  The  teeth  are  ranged  around 
the  margins  of  the  jaws  in  a  parabolic  cui*ve,  or  something 
approximating  to  one  ;  in  the  lower  races  of  mankind  the 
curve  tends  to  a  squarish,  oblong  form,  owing  to  the 
prominence  of  the  canines  (compare  the  figure  of  the  denti- 
tion of  Simia  Satyrus),  whilst  a  deviation  in  the  opposite 
direction  is  daily  becoming  more  common  in  the  most 
highly  civilised  races,  resulting  in  a  contour  to  which  in 
extreme  cases  the  name  of  V-shaped  maxilla  is  applied. 

It  may  be  stated,  as  generally  true,  that  the  teeth  are 
somewhat  larger  on  their  labial  than  on  their  lingual  aspect, 
a  result  which  necessarily  follows  from  their  standing  with- 
out interspaces  along  a  curved  line.  And  as  great  variations 
in  size  and  shape,  as  well  as  in  colour,  are  found  to  exist 
between  different  individuals,  it  is  only  possible  to  give 
such  a  description  as  shall  apply  to  the  generality  of  teeth. 

The  teeth  of  the  upper  jaw  are  ranged  along  a  curve  of 
larger  dimensions  than  those  of  the  lower,  the  incisors  pass- 
ing in  front  of  the  corresponding  lower  teeth,  and  the  ex- 
ternal cusps  of  the  bicuspids  and  molars  closing  outside 
those  of  the  lower  teeth. 

There  are,  however,  some  points  of  detail  to  be  noted  in 
the  relation  borne  by  the  upper  to  the  lower  teeth,  besides 
that  comprised  in  the  general  statement  that  the  former  lie 
outside  the  latter,  by  which  it  is  brought  about  that  each 
tooth  is  antagonised  by  portions  of  two  teeth  in  the  other 
jaw,  and  has  not  only  a  single  opponent. 

The  upper  incisors  and  canines,  when  the  mouth  is  closed, 
from  the  larger  size  of  the  arch  in  which  they  are  arranged, 


THE    TEETH    OF  MAN. 


shut  over  and  in  front  of  the  lower  teeth,  concealing  the 
upper  thirds  of  their  crowns  ;  while  the  external  tubercles 
of  the  bicuspids  and  molars  of  the  lower  jaw  are  received 
into  the  depressions  between  the  external  and  internal 
tubercles  of  the  similar  teeth  in  the  upper  jaw,  thus  alloM'- 
ing  the  external  tubercles  of  the  upper  teeth  to  close  ex- 
ternally to  the  outer  tubercles  of  the  lower  row. 

From  this  aiTangement  of  the  tubercles,  we  are  enabled 
in  mastication  to  use  the  wdiole  surface  of  the  crowns  of  the 
opposing  teeth ;  the  act  of  mastication  being  performed  by 
bringing  the  external  tubercles  of  the  under  molars  opposite 
to  those  of  the  upper  row ;  whence,  by  the  lateral  motion 
of  the  under  jaw^  inwards,  their  external  tubercles  pass  down 
the  inclined  surfaces  of  the  external,  and  up  those  of  the 
intei-nal  tubercles  of  the  upper  teeth,  crushing  in  this  action 
any  interposed  substance. 

It  will  also  l)e  observed  that,  from  the  difference  of  width 
in  the  incisors  of  the  two  jaws,  the  centi-al  incisors  of  the 
upper  extend  over  the  centrals  and  half  of  the  laterals  of 
the  under  row,  and  that  the  superior  laterals  lie  over  the 
remaining  half  of  the  inferior  laterals  and  the  anterior  half 
of  the  canines  of  the  lower  jaw.  The  canines  close  over  the 
halves  of  the  canines  and  first  bicuspids,  while  the  first  bi- 
cuspids impinge  on  the  half  of  the  first  and  half  of  the  second 
bicuspids  of  the  lower  row.  The  second  upper  bicuspids 
close  upon  the  anterior  third  of  the  opposing  first  molars 
and  the  posterior  half  of  the  second  bicuspids. 

The  first  molars  oppose  the  posterior  two  thirds  of  the 
first,  and  one  third  of  the  second  molars  of  the  lower  jaw, 
while  the  second  upper  molars  close  upon  the  unoccupied 
posterior  third  of  the  second  and  the  anterior  third  of  the 
wisdom  teeth.  The  wisdom  tooth  of  the  upper  being 
smaller  in  size  than  that  of  the  lower  jaw  is  perfectly 
opposed  by  that  portion  of  the  latter  left  unoccupied  by 
the  second  upper  molar  tooth. 


A    MANUAL    OF   DENTAL    ANATOMY. 


By  this  admirable  arraugement  no  two  teeth  oppose  each 
other  only,  but  each  tooth  in  closure  of  the  jaw  impinges 
upon  two,  so  that  should  a  tooth  be  lost,  or  even  two  alter- 
nate teeth,  still  the  corresponding  teeth  of  the  opposite  jaw 
are  to  some  extent  opposed,  and  thus  remain  useful.  For 
when  a  tooth  is  wholly  unopposed,  a  process  is  apt  to  be 
set  up  in  the  jaw  by  which  the  useless  organ  is  gradually 
ejected.  The  direction  of  the  teeth  in  the  upper  is  verti- 
cally downwards  and  slightly  forwards,  while  those  of  the 
lower  jaw  are  placed  vertically,  the  molars  tending  slightly 
inwards. 

It  is  usual  to  represent  the  dentition  of  any  animal  by 
what  is  termed  a  dental  formula,  which  enables  the  reader 
at  a  glance  to  see  the  number  of  teeth  of  each  variety  pos- 
sessed by  the  creature.  Thus,  instead  of  writing  out  at 
length  that  man  has  two  incisors  on  each  side  in  both  upper 
and  lower  jaws,  one  canine,  two  bicuspids  or  premolars,  &c., 
it  is  written  thus  : — 

T    2        1  2         3         „.^ 

I.  —  c.  —  prm.  —  m.  —  =  61 ; 
2        12  3 

or  in  the  deciduous  set : — 

I.  ^  c.  1  dm.  Ji  =  20. 

2        1  2 

For  the  purpose  of  description  three  parts  of  the  tooth 
are  distinguished  by  name,  viz.,  the  crown,  neck,  and  root. 

This  distinction  of  parts  which  we  make  in  describing 
human  teeth,  when  we  speak  of  crown,  neck,  and  root,  is 
applicable  to  the  great  majority  of  mammalian  teeth, 
though  there  are  some  few  simple  forms  of  teeth  in  which 
no  such  differentiation  of  parts  can  be  seen. 

The  crown  is  that  portion  which  is  exposed  above  the 
borders  of  the  gum,  and  is  in  human  teeth  coated  with 
enamel ;  the  neck  is  that  portion  which  corresponds  to  the 


THE   TEETH    OF  MAN. 


edge  of  the  gum,  and  intervenes  between  the  edges  of  the 
bony  sockets  and  the  edge  of  the  enamel ;  the  root  is  that 
part  -which  is  enclosed  within  the  bony  socket,  and  is 
covered  by  cementum. 

Of  these  it  is  to  be  remarked  that  the  "  neck,"  although 
a  convenient  and  necessary  term  for  descriptive  purposes, 
marks  an  arbitrary  division  of  less  importance  than  that 
expressed  by  crown  and  root ;  also  that  although  this  divi- 
sion into  thi-ee  parts  can  be  made  in  the  case  of  socketed 
teeth  of  limited  growth,  no  such  distinction  of  parts  can 
be  made  in  teeth  of  perpetual  growth. 

Special  names  have  been  applied  to  the  various  surfaces 
of  the  crow^ns,  as,  owing  to  the  curvature  of  the  alveolar 
border,  terms  which  had  reference  to  front,  back,  or  sides 
would,  in  different  parts  of  the  mouth,  indicate  difterent 
surfaces,  and  so  lead  to  confusion. 

The  lips  and  tongue  and  the  median  line  of  the  mouth, 
however,  are  not  open  to  this  objection,  so  the  sm-faces 
which  are  directed  outwards  towards  the  lips  are  called 
"  labial ; "  and  those  inwards  towai-ds  the  tongue  "  lingual ; " 
the  interstitial  sm-faces  are  called  "  median  "  and  "  distal," 
the  word  median  being  applied  to  the  surface  which  would 
look  towaixls  the  middle  line  of  the  mouth  had  the  alveolar 
border  been  straightened  out.  In  other  words  behind  the 
canine,  the  "  median  "  is  equivalent  to  anterior,  and  "  distal '' 
to  posterior  surface. 

Forms  of  the  several  Teeth. — It  is  usual  to  speak  of 
the  teeth  as  being  modified  cones,  and  to  attribute  their 
variations  to  deviations  from  this  typical  shape.  In  a  broad 
sense  this  may  be  true  of  the  simplest  teeth,  such  as  are  met 
with  in  some  fish  and  reptiles  and  monophyodont  mammals, 
which  are  little  more  than  simple  cones ;  but  there  are  in- 
dications which  would  point  to  something  more  complex 
than  this '  as  the  fundamental  form  of  a  mammalian  tooth, 
for  even  among  the  monophyodonts,  as  I  have  elsewhere 


8       A    MANUAL    OF   DENTAL    ANATOMY. 

pointed  out,  the  armadillo  has  a  bilobed  tooth  germ,  the 
one  cusp  predominating  over  the  other.  But  I  do  not  think 
that  we  have  at  present  the  data  upon  which  to  cer- 
tainly determine  the  fundamental  form  of  the  mammalian 
tooth. 

There  is  evidence  that  all  the  teeth  in  the  jaw  of  a 
mammal  may  have  been  derived  from  a  single  form ;  in 
other  words,  marked  though  the  distinction  between  in- 
cisors, canines,  bicuspids,  and  molars  seems  to  be  at  first 
sight,  a  closer  inspection  reveals  various  gradational  or 
transitional  characters  linking  them  together,  though  there 
are  gaps  in  the  chain  not  bridged  over  by  forms  known  to  us. 
This  may  be  seen  by  a  careful  study  of  the  human  teeth,  as 
I  shall  endeavour  to  show;  but  it  is  much  more  conspi- 
cuously seen  in  an  extinct  animal  (Homalodontotherium, 
an  extinct  ungulate  from  Patagonia,  described  by  Professor 
Flower,  Philos.  Trans.  1874),  which  ajDparently  possessed 
the  full  typical  number  of  mammalian  teeth,  viz.,  forty-four. 
The  point  in  which  its  dentition  is  chiefly  instructive  is  that 
the  teeth,  in  close  juxtaposition  one  with  another,  present 
an  exceedingly  perfect  gradation  of  form  from  the  front  to 
the  back  of  the  mouth,  no  tooth  differing  mai'kedly  from 
its  neighbour,  though  the  difference  between,  say,  the  first 
incisor  and  first  molar,  is  exceedingly  great.  In  Professor 
Flower's  words,  "  it  is  only  by  the  analogy  of  other  forms 
that  they  can  be  separated  into  the  groups  convenient  for 
descriptive  purposes,  designated  as  incisors,  canines,  premo- 
lars, and  molars." 

In  viewing  the  gradational  characters  which  do  exist  be- 
tween the  various  human  teeth,  it  must  not  be  forgotten 
that  some  links  in  the  chain  have  dropped  out  and  are 
absent.  Mention  has  already  been  made  of  the  full  typical 
number  of  mammalian  teeth  being  44,  i.e. 

T    3  1  4         3,, 

I.  _    c.    -  pnn.  _  m.  -    :=  44. 
3  1  4         3 


THE    TEETH    OF   MAN. 


The  human  subject  does  not  possess  the  third  incisor,  nor 
the  first  two  premohirs,  so  that  a  somewhat  abrupt  change 
of  form  in  passing  from  the  incisors  to  the  canines,  and 
from  the  latter  to  the  bicuspids,  is  no  more  than  might  be 
anticipated. 

Incisors. — Of  these  there  are  four  in  each  jaw ;  two  cen- 
tral, two  lateral  incisors.  Their  working  surfaces  form 
wedges,  or  obtuse  and  blunt-edged  chisels,  calculated  to 
divide  food  of  moderate  consistency. 

Upper  Incisors. — The  centrals  are  very  much  larger 
than  the  laterals,  and  viewed  either  from  the  back  or  front 
taper  with  some  regularity  from  the  cutting  edge  to  the 
point  of  the  I'oot,  the  neck  not  being  marked  by  strong  con- 
striction. The  crown  of  the  tooth,  as  seen  from  the  front, 
is  squarish,  or  more  strictly,  oblong,  its  length  being 
greater  than  its  breadth. 

The  median  side,  by  which  it  is  in  contact  with  its  fellow, 


Fig.  1  {') 


is  a  little  longer  than  the  distal,  so  that  the  median  angle  of 

the  crown  is  a  little  lower,  and,  as  a  necessary  consequence, 

a  little  more  acute  than  the  distal  angle  of  the  cutting  edge. 

Near  to  its  base  the  crow^ns  narrov/  rather  abruptly,  so  that 

near  to  the  neck  a  space  is  left   between  the  contiguous 

teeth. 

(')  Front  and  side  view  of  a  left  upper  central  incisor. 
a  Distal  surface.  6  Neck.  c  Koot. 


10  A    MANUAL    OF   DENTAL    ANATOMY. 

The  labial  sufface  is  slightly  convex  in  each  direction, 
and  often  presents  slight  longitudinal  depressions,  which  end 
at  the  cutting  edge  in  slight  notches. 

In  recently-cut  teeth  the  thin  cutting  edge  is  elevated 
into  three  slight  cusps,  which  soon  wear  down  and  disappear 
after  the  tooth  has  been  in  use. 

The  edge  of  an  incisor  may  be  regarded  as  formed  by  the 
bevelling  off  of  the  dentine  of  the  lingual  surface,  which  is 
nearly  flat  from  side  to  side,  with  a  slight  tendency  to  con- 
cavity, while  from  above  downwards  it  is  distinctly  concave, 
and  often  presents  longitudinal  depressions  similar  to  those 
on  the  labial  surface.  The  lingual  surface  towards  the  gum 
terminates  in  a  distinct  prominence,  oftentimes  amounting 
to  a  bounding  I'ing  of  enamel,  termed  the  basal  ridge,  or,  in 
the  language  of  comparative  anatomy,  the  cingulum.  It  is 
variable  in  the  extent  of  its  development ;  it  rarely  rises 
into  a  central  prominence  at  the  back,  but  in  the  angle 
where  the  ridges  of  the  two  sides  meet  a  deep  pit  is  often 
left  in  the  enamel,  Avhich  is  a  favourite  site  for  caries.  The 
crown,  or  what  amounts  to  the  same  thing,  the  enamel,  ter- 
minates on  the  lingual  and  labial  aspect  of  the  tooth  in  a 
curved  line,  the  convexity  of  the  curve  being  directed 
upwards  towards  the  ginn ;  on  the  insterstitial  surfaces, 
both  median  and  distal,  the  curve  is  less  regular,  and  its 
contour  would  be  more  correctly  described  as  V-shaped,  the 
apex  of  the  V  being  towards  the  crown  of  the  tooth  and 
away  from  the  gum.  The  dentist  wiU  do  well  to  remember 
the  disposition  of  the  enamel  in  this  situation,  as  it  is  a 
point  of  some  importance  in  shaping  the  cervical  edge  of  a 
cavity  preparatory  to  filling  it. 

The  transverse  indentations  of  the  enamel  met  with  both 
on  lingual  and  labial  surfaces,  though  more  especially  in  the 
lattei",  are  marks  of  an-est  of  development,  and,  common  as 
they  are,  are  to  be  regarded  as  abnormalities. 

The  centi-al  incisors  are  larger  than  the  laterals,  though 


THE   TEETH    OF   MAN. 


not  in  so  great  degree  as  is  the  case  in  the  anthropoid 
apes. 

The  pulp  cavity  bears  a  general  resemblance  to  the  ex- 
ternal contour  of  the  tooth ;  towards  the  cutting  edge  it  is 
very  thin,  and  is  prolonged  at  its  two  comers  to  a  slight 
extent  into  "  cornua ;"  at  the  neck  it  is  cylindrical,  and  is 
also  cylindrical  in  the  root,  tapering  gradually  till  it  ap- 
proaches close  to  the  apex,  when  it  becomes  suddenly  con- 
stricted. 

Upper  lateral  incisors  are  in  every  dimension  some- 
what smaller  than  the  centrals.  'They  widen  somewhat 
abruptly  near  to  the  cutting  edge,  but  below  this  they  taper 
pretty  regularly  to  the  end  of  the  root ;  the  labial  surface 


Fig.  2  ('). 


f      I 


is  convex  in  each  direction,  while  the  lingual  surface  is 
perhaps  rather  flatter  than  that  of  a  central  incisor. 

The  outer  (distal)  angle  of  the  crown  is  far  more  rounded 
or  sloped  away  than  in  the  centrals,  and  the  distal  siuface, 
looking  towards  the  canine,  is  in  a  slight  degree  convex; 
the  median  sui-fiice  may  be  slightly  concave. 

The  enamel  terminates  towai'ds  the  gum  in  contours  pre- 
cisely similar  to  those  which  obtain  in  the  centrals  :  but 
the  basal  ridge,  or  cingulum,  is  often  more  strongly  pro- 
nounced, and  the  presence  of  a  central  tubercle  upon  it  is 
less  infrequent.     From  this  greater  prominence  of  the  cin- 

(^)  Front  and  side  view  of  a  left  uiii^er  latei'al  incisor. 


12  A    MANUAL    OF  DENTAL    ANATOMY. 

gulum  and  consequent  more  marked  depression  in  front  of 
it,  caries  is  more  frequent  ujion  the  lingual  surfaces  of  upper 
lateral  than  upon  those  of  upper  central  incisors. 

The  pulp  cavity  is,  relatively  to  the  whole  tooth,  perhaps 
a  little  larger  than  in  the  central  incisors ;  in  other  respects 
the  same  description  will  suffice. 

Iiower  central  incisors  are  very  much  narrower  than 
those  of  the  upper  jaw ;  not  more  than  half  the  width  at 
their  cutting  edges,  which  again  ai-e  much  wider  than  the 
necks  of  the  teeth. 

From  before  backwards  they  are  deep  at  the  neck ;  hence 
the  fangs  are  very  much  flattened  from  side  to  side,  and 
rotation  is  inadmissible  in  the  attempt  to  extract  them. 

The  enamel  contour  at  the  neck  is  similar  to  that  of  the 
upper  incisors,  but  there  is  no  well-marked  cingulum. 

Fig.  3  ('). 


Iiower  lateral  incisors  are,  unhke  the  upper  teeth,  dis- 
tinctly larger  than  the  centrals  in  each  one  of  their  dimen- 
sions, but  more  especially  in  the  length  of  their  fangs, 
which  are  much  flattened,  and  often  present  on  their  sides 
a  median  longitudinal  depression,  sometimes  amounting  to 
an  actual  groove. 

The  distal  angle  of  the  crown  is  rounded  off  like  that  of 
the  upper  lateral  incisors,  though  not  so  markedly. 

Canines,  Cuspidati,  Eye  Teeth,  are,  in  all  respects, 

(')  Front  aud  side  view  of  lower  central  incisor. 


THE    TEETH    OF   MAN.  13 

stouter  teeth  than  the  incisors ;  not  only  are  the  crowns 
thicker  and  stronger,  but  the  roots  are  veiy  much  longer. 

The  crown  terminates  in  a  blunt  point,  which  lies  in  a 
straight  line  with  the  long  axis  of  the  root ;  a  feebly  pro- 
nounced line  or  ridge  runs  down  the  outer  surface  of  the 
tooth  from  this  point  to  the  neck.  The  crown  slopes  away 
both  before  and  behind  the  point  or  cusp,  but  as  that  side 
of  the  tooth  which  lies  next  to  the  bicuspid  is  convex,  and 
as  it  were  produced  towards  that  tooth,  the  slope  is  longer 
on  the  distal  than  on  the  mesial  half  of  the  crowai.  The 
croAvn  thus  not  being  perfectly  symmetrical  renders  it  easy 
to  determine  at  a  glance  to  which  side  of  the  mouth  the 
canine  belongs. 

The  internal  or  lingual  surface  is  not  concave  like  that  of 
the  incisors,  but  is  in  a  slight  degree  convex,  and  a  median 
ridge  runs  down  it  from  the  apex  of  the  cusp ;  this  ridge 

Fui.  4  ('). 


where  it  meets  with  the  ridge  which  borders  the  lingual 
surface  and  corresponds  with  the  cingulum  of  the  incisor 
teeth,  is  often  developed  into  a  well-marked  prominence  or 
cusp. 

In  transverse  section  the  neck  is  nearly  triangular,  the 
outer  or  labial  being  much  wider  than  the  lingual  aspect. 

(^)  Lingual;  labial,  and  distal  surfaces  of  an  upper  canine,  showing  the 
basal  cusp  and  the  three  ridges  which  converge  towards  it. 


14  A    MANUAL    OF   DENTAL    ANATOMY. 

Lower  canines  are  less  pi'onounced  in  form  than  the  cor- 
responding upper  teeth  :  the  point  is  more  blunted,  the 
fang  shorter,  the  perpendicular  labial  ridge  not  being 
traceable,  and  the  want  of  symmetry  between  the  mesial 
and  distal  halves  of  the  crown  less  marked.  The  lingual 
surface  has  perhaps  a  greater  tendency  to  concavity. 

Premolars,  Bicuspids,  are  eight  in  number,  two  on  each 
side  of  both  upper  and  lower  jaws,  and  they  correspond  to 
the  third  and  fourth  premolars  of  the  typical  mammalian 
dentition,  the  first  and  second  premolars  not  being  re- 
presented in  man. 

Upper  Premolars. — The  crown,  as  seen  looking  upon  its 
grinding  surface,  is  roughly  quadrilateral,  its  outer  or 
lingual  border  being,  however,  larger  and  thicker  than  its 
inner,  and  the  teeth  are  carried  round  the  curve  of  the 

Fig.  5  (i). 


alveolar  border  mainly  by  means  of  this  difference  in  size 
in  the  external  and  internal  portions  of  the  canines  and  the 
two  bicuspids. 

As  is  implied  by  its  name,  the  crown  has  two  cusps,  of 
which  the  outer  is  the  larger  and  stouter,  and  broader. 
The  outer  and  inner  surfaces  (labial  and  lingual)  are  convex 
and  smooth,  with  no  basal  ridges  at  the  edge  of  the  gums. 
The  inner  and  outer  cusps  arc  not  joined  by  a  transverse 
ridge ;  instead  of  this  there  is  a  deep  transverse  fissure  ;  in 
point  of  fact  the  cingulum  has  been  elevated  to  form  the 
inner  cusp,  and  forms  slight  elevations  bordering  the  anterior 

(^)  Grinding  surface  of  an  upper  bicuspid. 


THE    TEETH   OF  MAN.  15 

and  posterior  (mesial  aud    distal)    edges   of  the   grinding 
siu-face. 

The  root  is  single,  and  much  compressed  from  side  to 
side  :  very  often,  however,  it  is  double  for  the  greater  part 
of  its  length,  and  if  not  so  divided  is  often  marked  by  a 
groove  upon  each  side  indicating  a  tendency  towards  such 
division.  The  outer  border  of  the  root  is  also  often  marked 
by  a  longitudinal  fuiTow,  which  may  amount  to  complete 
division.  In  fact  a  bicuspid  may  have  three  perfectly  dis- 
tinct roots,  like  a  molar,  or  it  may  have  any  form  of  root 
intermediate  between  this  and  its  typical  single  laterally- 
flattened  root.  The  first  bicuspid  is  more  variable  in  respect 
of  its  roots  than  the  second. 

The  second  upper  bicuspid  differs  from  the  first  in  that 
the  diflference  in  size  between  its  outer  and  inner  cusps  is 
less,  the  inner  cusp  being  relatively  considerably  larger, 
and,  indeed,  often  preponderating  over  the  labial  cusp  in 
length. 

The  pulp  cavity  in  the  crown  is  furnished  with  distinct 
cornua ;  at  the  neck  it  is  very  much  flattened  from  side  to 
side,  being  often  reduced  to  a  mere  fissure,  which  is  how- 
ever considerably  larger  at  its  two  extremities  than  in  its 
middle.  Hence  the  pulp  cavity  of  an  upper  bicuspid  is 
difficult  to  fill ;  a  difl&cult^'^  again  increased  by  the  impos- 
sibility of  always  discovering  what  number  of  fangs  it  has, 
as  their  division  sometimes  takes  place  rather  high  up. 

lower  premolars  are  smaller  teeth  than  those  of  the 
upper  jaw,  and  are  quite  distinct  in  shape.  The  outer  or 
labial  cusp  is  bent  inwards,  and  the  labial  surface  of  the 
crown  is  very  convex.  The  inner  cusp  is  but  feebly  deve- 
loped, and  is  connected  with  the  outer  by  a  low  ridge ;  it  is 
also  narrow. 

The  root  is  rounded,  a  little  larger  on  its  outer  side  than 
on  its  inner,  and  tapers  regularly  towards  its  point ;  the 
pulp  cavity  is  cylindi'ical  at  the  neck,  and  also  tapers  regi>- 


16  A    MANUAL    OF  DENTAL    ANATOMY. 

larly  in  the  root.  The  coruu  of  the  pulp  which  corresponds 
to  the  inner  cusp  is  but  feebly  developed. 

The  second  lower  bicuspid  differs  a  good  deal  from  the 
first ;  its  crown  is  much  squarer  and  larger  in  all  its  dimen- 
sions. The  inner  cusp  reaches  to  a  higher  level  and  is 
stouter,  and  the  greater  development  of  the  ridge  which 
bounds  the  posterior  (distal)  border  of  the  grinding  surface 
makes  it  attain  to  such  a  large  size  as  to  make  the  tendency 
towards  a  transition  from  the  bicuspid  type  to  the  quadri- 
cuspid  type  of  a  true  molai-  very  evident. 

Having  completed  the  brief  description  of  the  forms  of 
these  several  teeth,  it  is  worth  while  to  note  one  or  two- 
general  characters  of  the  series.  The  differences  between  a 
well-marked  incisor,  canine,  or  premolar  are  so  strongly 
pronounced  that  the  resemblances  which  underlie  them  are 
apt  to  be  overlooked,  and  it  might  be  supposed  that  in  shape 
they  had  little  in  common. 

Nevertheless  a  very  distinct  gradation  may  be  traced,  and 

Fig.  6  ('). 


it  is  far  from  uncommon  to  meet  with  teeth  which  possess 
in  a  marked  degree  transitional  characters.  If  the  external 
or  distal  angle  of  a  lateral  incisor  be  sloped  off  more  than 
usual,  while  at  the  same  time  its  cingulum  and  basal  pro- 
minence be  well  marked,  it   makes  no  bad  imitation  of  a 

(1)  Lower  first  bicuspid,  seen  from  the  iuner  side,  and  sliowiiig  the  pre- 
ponderance of  its  outer  over  its  inner  cusp. 


THE    TEETH    OF    MAN.  17 

diminutive  canine  ;  and  such  laterals  ai'e  often  to  be  met 
with  by  any  who  search  for  such  deviations  from  the  normal 
form. 

Thus  the  form  chai'acteristic  of  a  lateral  incisor,  if  it  be  a 
little  exaggerated,  very  nearly  gives  vis  the  form  of  a  canine, 
and  if  we  look  at  the  teeth  of  an  Orang  the  lateral  incisor  is 
to  all  intents  a  diminutive  canine  ;  and  in  the  present  dis- 
cussion the  great  comparative  size  of  the  canine,  which  is 
traceable  to  readily  intelligible  causes,  may  be  put  aside,  as 
it  tends  to  obscure  the  point  to  be  here  insisted  on. 

Between  the  canines  and  the  bicuspids  a  similar  relation- 
ship in  form  exists,  and  it  is  more  apparent  in  the  lower 
than  in  the  upper  jaw.  The  fact  that  at  the  base  of  the 
inner  or  lingual  aspect  of  the  canine  is  to  be  found  an 
elevation  of  the  cingulum,  in  many  instances  amounting  to 

Fia.  7  ('). 


a  low  cusp,  has  been  already  noted ;  and  it  has  already  been 
pointed  out  that  the  inner  cusp  of  the  first  lower  bicuspid  is 
both  smaller  and  lower  than  the  outer,  A  longitudinal 
section  through  the  crowns  of  the  two  teeth  will  demon- 
strate without  the  necessity  of  further  description  that  the 

(')  Section  of  a  lower  canine  and  first  bicuspid,  showing  the  characteiB 
common  to  the  two. 

C 


18  A    MANUAL    OF   DENTAL   ANATOMY. 

basal  cusp  of  the  canine  and  the  mner  cusp  of  the  bicuspid 
are  the  same  thing,  differing  only  in  degree,  while  it  is 
interesting  to  note  that  the  pulp  chamber  in  the  bicuspid 
has  hardly  any  prolongation  towards  the  small  inner  cusp, 
so  that  the  resemblance  between  the  two  teeth  is  thus  made 
more  complete. 

This  close  relationship  of  canines  and  bicuspids  will  be 
again  considered  in  the  chapter  on  the  Homologies  of  the 
Teeth ;  for  our  present  purpose  it  will  suffice  to  merely  point 
out  its  existence.  The  transition  from  the  bicuspids  to  the 
molars  is  more  abrupt ;  at  least  it  is  not  so  easy  to  point 
out  exactly  how  a  modification  of  the  one  would  arrive  at 
the  form  of  the  other.  But  it  merely  needs  an  exaggera- 
tion of  the  differences  existing  between  a  canine  and  a  first 
bicuspid  to  make  a  good  imitation  of  a  second  bicuspid. 

If  any  one  will  take  the  trouble  to  make  mental  note  of 
the  deviation  in  form  wdiich  he  meets  with  in  teeth,  he  will 
find  that  they  almost  invariably  consist  of  approaches  to- 
wards the  form  of  the  teeth  on  either  side  of  them ;  and 
will  infallibly  be  led  to  the  conclusion  that  incisors,  canines, 
and  bicuspids  are  not  three  patterns  of  teeth  perfectly 
distinct,  and  each  sid  generis,  but  that  they  are  modifications 
of  one  and  the  same  pattern.  I  may  add,  that  comparative 
odontology  teaches  us  the  same  thing,  and  demonstrates 
clearly  the  substantial  identity  of  the  three  forms,  as  also 
of  the  true  molars. 

Upper  molar  teeth  have  crowns  of  squarish  form,  the 
angles  being  much  rounded  off  It  may  be  premised  that 
the  first  molar  is  more  constant  in  shape  than  the  second, 
and  this  latter  than  the  third ;  with  this  proviso  the  first 
and  second  may  be  described  together. 

The  masticating  surface  carries  four  subeqiial  cusps,  two 
labial  or  external  and  two  lingual  or  internal ;  the  anterior 
internal  cusp  is  distinctly  the  largest,  and  it  is  connected 
with  the  posterior  external  cusp  by  a  thick  oblique  ridge 


THE    TEETH    OF  MAN. 


of  enamel,  the  remaining  two  cus^^s  having  no  such  con- 
nection. 

This  oblique  ridge  on  the  upper  molars  is  met  with  in 
man,  the  anthropoid  apes,  and  certain  New  World  monkeys. 

The  grooves  which  separate  the  cusps  pass  down  on  to 
the  labial  and  lingual  surfaces  of  the  crown,  but  are  lost 
before  reaching  the  gum ;  where  they  terminate,  however, 
there  is  often  a  pit,  which  is  a  very  favourite  situation  for 


Fig.  8  (i). 


caries,  especially  on  the  labial  aspect  of  the  teeth.  It  ia 
very  rare  to  see  the  grooves  passing  down  upon  the  mesial 
or  distal  surfaces  of  the  crown,  a  raised  border  of  enamel 
generally  cutting  them  short  in  this  direction. 

The  roots  are  three  in  number,  two  external  or  labial,  and 
one  internal  or  palatal.  The  latter  is  the  largest,  and  runs 
in  a  direction  more  strongly  divergent  from  the  axis  of  the 
crown  than  the  other  roots.  It  is  directed  obliquely  in- 
wards towards  the  roof  of  the  palate,  is  subcylindrical,  and 
often  curved. 

The  external  roots  are  less  cylindrical,  being  mutually 
compressed,  so  that  their  largest  diameter  is  transverse  to 
the  dental  arch  ;  the  anterior  is  rather  the  larger  of  the  two, 
and  is  more  strongly  pronounced  on  the  side  of  the  neck  of 
the  tooth.     The  anterior  labial  root  is  occasionally  confluent 

(^)  Masticating  surface  of  a  first  upper  molar  of  the  left  side  ;  the 
oblique  ridge  connects  the  anterior  internal  with  the  posterior  external 
cusp. 


20  A    MANUAL    OF   DENTAL    ANATOMY. 

with  the  palatine  root,  but  still  more  freqiiently  the  pos- 
terior labial  and  palatine  roots  are  confluent  :  occasionally, 
also,  four  distinct  roots  may  be  met  with. 

Lower  molars. — The  first  loAver  molar  is  the  most  con- 
stant in  form,  and  is  somewhat  the  largest ;  its  grinding 
surface  presents  five  cusps. 

Four  cusps  are  placed  regularly  at  the  four  corners  of  a 
square,  these  being  divided  from  one  another  by  a  crucial 
fissure ;  the  posterior  arm  of  the  crucial  fissure  bifurcates, 
and  between  its  diverging  arms  is  the  fifth  cusp,  which  is 
thus  to  be  described  as  median  and  posterior. 

Fig.  0  (1), 


The  transverse  fissure  passes  over  the  limits  of  the  grind- 
ing surface,  and  on  the  outside  or  labial  siu-face  of  the  tooth 
ends  in  a  pit,  which  is  a  common  site  for  caries ;  although  it 
occasionally  passes  over  the  lingual  siirface,  it  is  here  less 
pronounced.  Tliey  are  implanted  by  two  fangs,  jilaced 
anteriorly  and  posteriorly  ;  the  roots  are  much  flattened 
from  before  backwards,  and  they  are  very  usually  curved 
slightly  backwards.  In  the  median  line  of  each  root  there 
is  usually  a  groove,  by  the  deepening  of  which  four  fangs 
may  be  produced  ;  or  this  may  happen  with  the  one  root 
only,  so  that  a  three  rooted  tooth  is  the  result. 


Q)  Masticating  surface  of  a  first  lower  molar,  riglit  side,  the  five  cusps 
of  which  are  indicated  by  figures. 


THE    TEETH    OF   MAN. 


The  second  Biolar  does  not  greatly  differ  from  the  first 
save  that  the  roots  are  more  often  confluent,  and  the  fifth 
cusp  less  marked,  even  if  it  exists  at  all. 

Fig.   10  (1). 


Third  molars,  denies  sapientia^,  wisdom  teeth,  of  the  upper 
jaw,  resemble  in  a  general  way  the  first  and  second  molars  ; 
that  is,  when  they  are  well  developed  and  placed  in  a  roomy 
dental  arch.  But  amongst  more  civilised  races  it  may 
almost  be  said  to  be  exceptional  for  the  wisdom  teeth  to  be 
vegidar  either  in  form  or  position,  so  that  extreme  varial)ility 
prevails  among  these  teeth. 

The  two  inner  tubercles  are  often  l)lended  together  and 
the  roots  confluent,  forming  an  abruptly  tapering  cone,  the 
apex  of  which  is  often  bent  and  crooked,  so  that  but  little 
vestige  of  the  three  roots  can  be  traced,  the  pulp  cavity  even 
being  quite  single. 

Third  lower  molar. — This  tooth  is  seldom  so  small  as 
the  corresponding  upper  tooth,  and  its  crown  is  often  large 
even  w^hen  its  roots  are  very  stunted.  It  has  five  cusps  as 
a  rule,  and  bears  a  more  or  less  close  resemblance  to  the 
molars  which  precede  it.  It  is  either  two-rooted,  or  if  the 
roots  be  confluent,  a  groove  usually  marks  a  tendency  to 
division  into  two  fangs. 

It  is  stated  by  Prof.  Owen  ("  Odontography,"  page  454) 
that  although  the  wisdom  tooth  is  the  smallest  of  the  three 
molars,  the  difference  is  less  marked  in  the  Melanian  than  in 

(1)  Second  lower  molar  of  right  side,  the  four  cusps  being  indicated  by 
figures. 


22  A    MANUAL    OF   DENTAL    ANATOMY. 

the  Caucasian  races,  adding  also  that  the  trii)le  implantation 
of  the  upper  and  the  double  implantation  of  the  lower  is 
constant  in  the  former  races.  More  extended  observations 
have  overthrown  this  statement  as  a  positive  dictum  to  bo 
accepted  without  excejjtions,  but  it  may  nevertheless  bo 
taken  as  expressing  a  general  truth. 

Fig.  11  (1). 


The  milk  teeth  differ  from  the  permanent  teeth  by  being 
smaller,  and  having  the  enamel  terminating  at  the  neck 
with  a  thick  edge,  so  that  the  neck  is  more  distinctly  con- 
stricted. The  incisors  and  canines  are  somewhat  similar  to 
their  successors,  the  canines,  however,  being  relatively  shorter 
and  broader  than  their  successors.  The  first  upper  molars 
have  three  cusps,  two  external  and  one  internal :  the  second 
more  nearly  resemble  the  permanent  molars. 

The  second  lower  deciduous  molar  has  four  cusps  and 
resembles  a  second  lower  permanent  molar.  The  roots  of 
the  deciduous  teeth  diverge  from  the  neck  at  greater  angles 
than  those  of  permanent  teeth,  in  consequence  of  their  mor6 
or  less  completely  enclosing  between  them  the  crypts  in 
which  the  latter  are  developing. 

(^)  Third  lowei'  molar  of  the  left  side. 


CHAPTER   II. 


THE    M.VXILLARY    BONES. 


The  teeth  are  implanted  iu  a  part  of  the  jaw  bones 
specially  developed  for  the  piu'pose,  the  bone  being  moulded 
around  the  roots  of  the  teeth  subsequently  to  their  being 
formed  and  moved  into  position. 

The  manner  of  attachment  of  the  human  teeth  is  that 
termed  "gomphosis,"  i.e.,  an  attachment  comparable  to  the 
fitting  of  a  peg  into  a  hole ;  the  bony  sockets,  however, 
allow  of  a  considerable  degree  of  motion,  as  may  be  seen  by 
examining  the  teeth  in  a  dried  skull,  the  fitting  being  iu 
the  fresh  state  completed  by  the  interposition  of  the  dense 
periosteum  of  the  socket.  This  latter,  by  its  elasticity, 
allows  of  a  small  degree  of  motion  in  the  tooth,  and  so 
doubtless  diminishes  the  shock  which  would  be  occasioned 
by  mastication  were  the  teeth  perfectly  immovable  and 
without  a  yielding  lining  within  their  bony  sockets.  When 
this  becomes  inflamed  and  swollen  by  exudation  the  tooth  is 
pushed  to  a  certain  extent  out  of  the  socket,  and  so  being 
to  a  less  extent  limited  in  its  range  by  the  bony  socket, 
acquires  an  increased  mobility. 

The  teeth  are  in  all  mammalia  confined  to  the  bones  which 
carry  them  in  man,  namely,  the  intermaxillary  and  maxillary 
bones  and  the  lower  maxillaiy  bone  or  mandible. 


24  A    MANUAL    OF  DENTAL    ANATOMY.    . 


While  full  description  of  these  bones  (>)  will  be  found  in  any 
general  anatomical  work,  there  are  so  many  points  in  their 
anatomy  which  directly  concern  the  dental  studeiit  that  a 
brief  enumeration  of  some  of  their  relations  can  hardly  be 
dispensed  with, 

Superior  maxillary  bone. — To  facihtate  description  of 
its  parts,  anatomists  divide  it  into  a  "body"  and  "pro- 
cesses," of  which  latter  there  are  four,  the  nasal,  malar, 
alveolar,  and  palatine.  As  the  body  of  the  bone  is  hollowed 
out  by  an  air  cavity,  the  antrum,  its  shape  is  similar  to  that 
of  that  cavity,  namely,  roughly  pyramidal,  the  base  of  the 
pyramid  being  inwards  towards  the  nasal  chamber. 

The  nasal  process  springs  directly  upwards  from  the  body 
in  a  vertical  line  with  the  canine  tooth  :  it  is  a  strong  plate 
of  bone,  roughly  triangular  when  viewed  from  the  side. 

The  malar  process  forms  the  apical  portion  of  the  pyramid 
already  alluded  to  ;  it  starts  out  nearly  horizontally  from 
the  body  just  behind  and  below  the  nasal  process,  and  is 
characterized  by  its  great  strength  and  stoutness.  Never- 
theless it  has  been  known  to  be  fractared  by  a  blow,  and 
separated  from  the  body  of  the  bone.  The  antrum  may  be 
prolonged  into  it. 

The  palatine  process  foi'ms  a  horizontal  table  projecting 
inwards  from  the  body;  as  the  floor  of  the  nose  is  nearly  flat, 
and  the  palate  is  arched  from  before  backwards,  the  front  of 
the  palatine  process  is  necessarily  much  thicker  than  the 
back,  where  it  is  quite  a  thin  plate. 

The  alveolar  process  is  a  strong  wide  ridge  of  bone,  curved 
so  as  to  form  with  that  of  the  other  maxillary  bone  the 
elliptical  figure  characteristic  of  the  dental  arch  in  the  higher 
races.  It  may  be  described  as  consisting  of  two  j^lates,  an 
outer  and  an  inner,  which  arc  connected  by  numerous  trans- 

(*)  Much  that  is  of  great  interest,  and  that  is  not  to  be  found  in  text 
books,  is  embodied  in  a  series  of  papers  on  "  The  Facial  Region,"  by  Dr. 
HaiTison  Allen  (American  Dental  Cosmos,  1873-7-1). 


THE   MAXILLARY   BONES.  25 

verse  septa,  the  sockets  of  the  teeth  being  formed  by  the 
interspaces  between  these  septa.  The  internal  alveolar  plate 
is  the  stronger,  the  external  the  thinner  and  weaker,  a  fact 
of  which  we  take  advantage  when  we  extract  a  tooth  by- 
bending  it  slightly  outwards.  On  the  outer  surface  of  the 
alveolar  process  are  eminences  corresponding  to  the  roots 
of  the  teeth,  and  depressions  in  their  interspaces,  apt  to  bo 


especially  marked  over  the  canine  teeth  ;  while  between 
the  teeth  the  alveolar  processes  attain  to  a  lower  level,  so 
that  the  margins  of  the  bone  are  festooned.  Looking 
down  into  an  empty  socket,  the  bone  is  seen  to  be  every- 
where very  porous,  and  to  be  perforated  by  foramina  of 
considerable  size,  while  at  the  bottom  there  is  the  larger 
foramen  admitting  the  vessels  and  nerves  of  the  tooth. 

The  alveolus  of  each  individual  tooth  consists  of  a  shell  of 
comparatively  dense  bone  of  small  thickness,  which  is  im- 
bedded in  a  mass  of  loose  spongy  bone;  this  dense  shell 
comes  into  relation  with  the  dense  cortical  bone  of  the  jaw 

(')  Superior  maxillary  boue  of  riglit  side.  1.  Boily.  2.  Tuberosity. 
7.  JIalar  process.     8.   Nasal  process.     12.   Alveolar  process. 


26  A    MANUAL    OF  DENTAL    ANATOMY. 

mainly  at  its  free  margin,  near  to  the  neck  of  the  tooth. 
Over  very  prominent  roots  a  portion  of  alveolus  is  at  times 
"wanting,  so  that  in  a  macerated  skull  the  root  is  exposed  to 
view. 

The  upper  maxilla  serves  to  give  form  and  support  to  the 
soft  parts  of  the  face,  and  also  to  carry  the  upper  teeth. 
These  have  to  be  rigidly  fixed,  while  the  teeth  of  the  lower 
jaw  are  brought  forcibly  against  them  with  more  or  less  of 
shock.  And  whilst  these  blows  have  to  be  received,  and 
resisted,  and  ultimately  borne  by  the  cranium,  it  is  obviously 
desirable  that  they  should  be  disti-ibuted  over  a  sufficiently 
wide  area,  so  as  not  to  be  felt  unpleasantly. 

The  ascending  nasal  process  is  very  stout,  and  serves  to 
connect  the  maxilla  strongly  with  the  frontal  bone,  which 
also  in  the  region  in  question  is  powerfully  developed  ;  the 
thick  malar  process  gives  rigidity  and  resistance  to  lateral 
movements  of  the  jaws,  and  carries  off  the  strains  to  the 
lateral  walls  of  the  cranium  ;  it  is  buttressed  at  the  back 
by  the  pterygoid  processes. 

Taking  next  the  various  surfaces  of  the  bone,  there  are 
four,  or,  if  we  include  the  palatine  aspect,  five :  the  external, 
forming  a  large  part  of  the  face,  the  superior  or  orbital,  the 
internal  or  nasal,  and  the  posterior  or  zygomatic.  Upon  the 
external  or  facial  surface  we  have  to  note  the  eminence 
caused  by  the  socket  of  the  canine  tooth  ("  canine  emi- 
nence"), and  immediately  behind  this  a  depression,  the 
canine  fossa,  through  which  the  antrum  is  sometimes  punc- 
tured. The  alveolar  border,  from  the  situation  of  the  third 
molar  to  that  of  the  second  bicuspid,  gives  attachment  to 
the  buccinator  muscle ;  while  immediately  beneath  the 
margin  of  the  orbit  is  the  infra-orbital  foramen,  whence 
issues  the  infra-orbital  nerve ;  hence  this  is  one  of  the  situa- 
tions to  which  neuralgic  pain  really  dependent  on  the  teeth 
may  be  referred. 

The  orbital  and  nasal  surfaces  concern  us  only  through 


THE   MAXILLARY   BOXES. 


their  relation  to  the  autrura,  to  be  presently  described  ;  in 
the  zygomatic  sui'face,  which  is  convex  and  forms  part  of 
the  zygomatic  fossa,  are  several  orifices  transmitting  the 
posterior  dental  nerves  and  vessels  ;  a  groove  which,  con- 
nected by  the  apposition  of  the  palate  bone  into  a  canal, 
forms  the  posterior  palatine  canal ;  and  at  the  bottom,  a 
rounded  eminence,  the  maxillary  tuberosity,  -which  lies 
behind  the  wisdom  tooth,  and  has  been  occasionally  broken 
off  in  extracting  that  tooth. 

The  body  of  the  bone  is  excavated  by  an  air-chambei',  the 
antrum,  which  is  coated  in  life  by  a  continuation  of  the  nasal 
mucous  membrane,  and  this  frequently  becomes  secondarily 
involved  in  dental  disease,  so  that  its  anatomical  relations 
are  of  great  importance  to  the  dentist. 

Like  the  somewhat  similar  air  cavities  in  the  frontal  bone 
the  maxillary  sinus  does  not  attain  to  its  full  size,  relatively 
to  the  rest  of  the  bone,  until  after  the  age  of  puberty, 
although  it  makes  its  appearance  earlier  than  the  other 
nasal  sinuses,  its  presence  being  demonstrable  about  the 
fifth  month  of  foetal  life.  Hence  it  follows  that  its  walls  are 
thicker  in  the  young  subject  than  in  the  adult ;  and,  ac- 
cording to  the  observations  of  Mr.  Cattlin  {^),  it  is  somewhat 
larger  in  the  male  than  in  the  female. 

It  is  very  variable  in  size,  so  that  out  of  one  hundred 
adult  specimens  the  above-mentioned  writer  found  one 
which  would  only  contain  one  drachm  of  fluid,  while  in 
contrast  with  that  was  another  which  held  eight  drachms  ; 
two  and  a  half  drachms  being  the  average  capacity.  Al- 
though it  is  exceedingly  variable  in  form  as  well  as  in  size, 
it  tends  towards  a  roughly  pyramidal  shape,  the  apex  of  the 
pyramid  being  directed  towards  the  malar  bone,  which  it 
has  been  seen  to  encroach  upon,  and  the  base  towards  the 
nasal  cavity;  it  is,  however,  useless  to  minutely  describe 

(')   "Transactions  of  tlie  Odontological  Society,"  vol.  ii.  1857. 


A    MANUAL    OF  DENTAL    ANATOMY. 


its  form,  inasmuch  as  the  two  antra  in  the  same  individual 
are  sometimes  quite  dissimilar.  The  floor  of  the  cavity  is 
rendered  uneven  in  most  specimens  by  prominences  corre- 
sponding to  the  roots  of  the  molar  teeth,  which  ordinarily 
are  but  thinly  covered  by  its  bony  walls,  while  it  is  not  by 
any  means  rare  to  find  some  of  them  actually  bare. 

The  cavity  is  also  more  or  less  completely  subdivided  by 
loony  partitions  springing  from  its  walls,  as  is  well  exempli- 
fied in  the  accompanying  figure ;  these  partitions  are  for 
the  most  part  thin,  but  they  occasionally  attain  to  consider- 
able thickness,  and  they  are  stated  to  occur  most  frequently 
at  the  anterior  or  posterior  angles  of  the  base  of  the 
pyramid. 

On  the  base  of  the  pyramid  is  the  orifice  by  which  it 
opens  into  the  middle  meatus  of  the  nose ;  this  orifice 
being  partly  closed  in  by  the  ethmoid,  palate,  and  inferior 
turbinated  bones,  and  also  by  soft  parts,  so  that  in  a  recent 

Fig.  13  ('). 


subject  it  will  barely  admit  a  goosequill ;  and  it  should  be 
noted  that  this  orifice  opens  into  the  antrum  near  the  top, 
so  that  it  does  not  afibrd  a  ready  means  of  egress  to  fluids 
accumulated  in  the  cavity. 

Through  this  orifice  the  mucous  membrane  Hning  the 

(1)  Section  of  an  antmm  of  the  left  side,  divided  into  many  iwuches, 
ijy  bony  septa,  and  extending  into  the  malar  bone.  Drawn  from  a 
specimen  in  the  collection  of  Dr.  Maynard,  in  the  possession  of  the  Bal- 
timore Dental  College. 


THE    MAXILLARY   BONES. 


antrum  is  contiuuous  with  that  of  the  nasal  fossa?,  and, 
like  that,  it  is  ciliated;  but  it  differs  from  the  latter  in 
being  thinner  and  less  vascular. 

The  teeth  which  usually  come  into  the  closest  relation 
with  the  antrum  are  the  first  and  second  molars,  but  any  of 
the  teeth  situated  in  the  maxillary  bone  may  encroach  upon 
its  walls,  and  I  have  seen  an  abscess,  originating  at  the  apex 
of  the  fang  of  a  lateral  incisor,  pass  backwards  and  perforate 
the  antrum. 

Its  walls  have  four  aspects,  namely,  toAvards  the  orbit, 
the  nose,  the  zygomatic  fossa,  and  the  face,  while  its  floor  is 
formed  by  the  alveolar  border.  With  the  exception  only  of 
the  latter,  its  walls  are  very  thin ;  and  this  exception  has 
an  important  practical  bearing  in  the  diagnosis  of  tumors  in 
this  region,  as  accumulations  of  fluid  or  morbid  growths 
really  situated  in  the  antrum  bulge  any  or  all  of  its  walls 
in  preference  to  the  alveolar  l:)order,  whereas  tumors  sjiring- 
ing  from  the  base  of  the  sphenoid  or  elsewhere  and  encroach- 
ing upon  the  antnim,  push  down  and  distort  the  alveolar 
border  as  easily  as  any  of  the  other  walls  of  the  cavity, 
inasmuch  as  the  pressure  caused  by  them  is  not  transmitted 
equally  in  all  directions,  as  is  the  case  when  the  medium 
transmitting  the  power  is  a  fluid. 

The  lower  maxilla  or  mandible  consists  of  a  body  and 
two  rami,  which  ascend  almost  pei-peudicularly  from  its 
posterior  extremity.  The  horizontal  portion  or  body  is 
curved  somewhat  in  a  parabolic  form ;  it  has  a  convex  ex- 
ternal and  concave  internal  surface,  and  an  ixpper  (alveolar) 
and  a  lower  border.  On  the  convex  facial  surface  we  have 
to  note  the  ridge  marking  the  position  of  the  symphysis, 
and  below  this  the  mental  prominence.  Externally  to  this, 
below  the  line  of  contact  of  the  first  and  second  bicuspids 
(or  a  little  before  or  behind  this  point)  is  the  mental  fora- 
men, which  constitutes  the  termination  of  the  inferior 
dental  canal.     Running  obliquely  upwards,  and  first  visible 


30 


A    MANUAL    OF  DENTAL    ANATOMY. 


at  a  point  a  little  distance  from  the  mental  prominence  is 
the  external  oblique  line,  which  becomes  merged  in  the  base 
of  the  coronoid  process.  Where  it  rises  as  high  as  the 
alveolar  border,  i.e.,  opposite  to  the  third  and  sometimes  the 


Fig.  14  (ij. 


second  molar,  the  outer  alveolar  plate  is  strengthened  by  it, 
so  that  it  becomes  less  yielding  than  the  inner  plate.  The 
student  should  bear  this  fact  in  mind  when  extracting  a 
lower  wisdom  tooth. 

The  buccinator  is  attached  to  the  alveolar  border  oppo- 
site to  the  molar  teeth  ;  the  platysma  myoides  to  the  outer 
side  of  the  lower  border  along  a  region  somewhat  further 
forward  :  the  masseter  over  the  whole  outer  face  and  border 
of  the  ascending  ramus  and  the  temporal  to  the  apex  and 
side  of  the  coronoid  process.  The  other  muscles  attached 
to  it  are  facial  muscles  of  expression. 

On  the  inner  surface  of  the  body  are  four  tubercles, 
situated  in  pairs  in  the  median  line,  about  opposite  to  the 
ends  of  the  roots  of  the  incisors,  but  somewhat  variable  both 


(^)  Lower  Maxillary  Bone.  2.  Eamus,  where  masseter  is  attached. 
3.  Symphysis.  5.  Mental  foramen.  6.  External  oblique  line.  8.  Angle 
of  jaw.  9.  Internal  oblique  line.  10.  Coronoid  process.  11.  Condyle. 
12.  Sigmoid  notch.     13.  Inferior  dental  foramen. 


THE   MAXILLARY  BOXES. 


in  position  and  in  size  in  different  individuals.  The  upper 
pair  of  tubercles  give  attachment  to  the  genio-hyo-glossus, 
the  lower  to  the  genio-hyoid  muscles  ;  they  are  interesting  to 
the  dental  student  not  only  as  giving  attachment  to  muscles 
conceraed  in  deglutition,  but  as  affording  convenient  fixed 
points  for  measurements  of  the  relative  growth  of  pai'ts  of 
the  jaw.  Beneath  these  genioid  tubercles  lie  the  slight 
depressions  which  give  attachment  to  the  anterior  belly  of 
the  digrastric  muscle,  while  between  the  two  points  alluded 
to  commences  the  interaal  oblique  line,  which  runs  ob- 
liquely upwards  and  backwards,  becoming  more  pronounced 
as  it  extends  backwards,  and  terminating  at  the  inferior 
dental  foramen.  This  internal  oblique  ridge  marks  the  line 
of  gi'owth  of  the  condyle  (see  Development  of  the  Jaws),  and 
gives  attachment  to  the  mylohyoid  muscle,  which  forms  the 
floor  of  the  mouth,  in  all  its  length.  Thus  the  bone  above 
the  ridge  belongs  strictly  to  the  mouth,  that  below  it  has 
more  relation  with  cervical  structures.  The  depression  for 
the  sublingual  gland  is  above  this  line,  consequently  this 
gland  is  visible  from  the  mouth ;  that  for  the  submaxillary 
gland  is  beneath  it  and  further  back. 

The  inner  surface  of  the  ascending  ramus  gives  attach- 
ment to  the  following  muscles  :  at  the  neck  of  the  condyle 
to  the  external  pterygoid ;  on  the  inner  face  of  the  coro- 
noid  process,  as  far  down  as  the  level  of  the  top  of  the 
crown  of  the  wisdom  tooth,  to  the  temporal ;  on  the  inner 
side  of  the  angle,  over  a  large  surface,  to  the  internal 
pterygoid. 

The  orifice  of  the  inferior  dental  canal  is  rough  and 
spinous,  giving  attachment  to  the  internal  lateral  ligament 
of  the  jaw,  while  beneath  and  behind  it  is  the  groove  for 
the  mylohyoid  vessels  and  nerves ;  the  canal  runs  forward 
in  the  bone  a  little  distance  beneath  the  ends  of  the  roots  of 
the  teeth,  and  emerges  at  the  mental  foramen,  turning  out- 
wards at  an  angle  to  reach  it,  and  sending  onwards  small 


32  A    MANUAL    OF  DENTAL    ANATOMY. 

cauals  to  the  incisors,  not  traceable  far.  It  is  nearer  to  the 
enter  than  to  the  inner  surface  of  the  jaw  in  the  latter  half 
of  its  course,  and  is  apt  to  be  very  close  to  the  ends  of  the 
roots  of  the  wisdom  teeth,  and  to  those  of  the  bicuspids. 
The  alveolar  processes  of  the  lower  jaw,  at  their  posterior 
part,  diverge  more  widely  than  those  of  the  ixpper  jaw,  the 
relative  antagonism  between  the  upper  and  lower  teeth 
being  preserved  in  this  region  by  the  former  having  an  in- 
clination outwards,  the  latter  inwards.  The  ascending  rami 
join  the  body  at  an  angle  which  is  very  obtuse  in  the  foetus, 
nearly  a  right  angle  in  the  adult,  and  once  again  obtuse  in 
advanced  old  age ;  the  explanation  of  this  change  will  be 
given  \inder  the  head  of  the  Development  of  the  Jaw. 

The  articulation  of  the  human  lower  jaw  is  peculiar,  and 
allows  of  a  degree  of  play  unusual  in  a  joint.  The  ovoid 
condyles,  when  the  jaw  is  at  rest,  are  lodged  in  depressions, 
the  glenoid  fossce  of  the  temporal  bone,  formed  partly  by  the 
squamous  and  partly  by  the  vaginal  portions  of  the  bone. 
The  posterior  half  of  the  cavity  is  rough,  and  lodges  a  portion 
of  the  parotid  gland  :  the  anterior  is  smooth,  and  is  bounded 
in  front  by  the  eminentia  articularis,  which  is  the  middle 
root  of  the  zygoma,  enters  into  the  formation  of  the  joint, 
and  is  coated  over  by  cartilage.  Between  the  condyle  of  the 
lower  jaw  and  the  temporal  bone  lies  a  moveable  inter- 
articular  fihro-cartilage,  which  is  an  irregular  bi-concave  oval 
plate,  the  edges  of  which  are  united  with  the  capsular  liga- 
ment, so  that  the  joint  is  divided  into  two  cavities,  furnished 
with  separate  synovial  membranes  (unless  when,  as  some- 
times is  the  case,  the  fibro-cartilage  is  perforated  in  its 
centre). 

The  joint  is  described  as  having  foiu-  ligaments  :  the 
capsular,  stylo-maxillary,  internal  and  external  lateral 
ligaments. 

The  capsular  ligament  is  but  feebly  pronounced,  and 
hai-dly  deserves  the  name  ;  the  stylo-maxillary  reaches  from 


THE   MAXILLARY  BONES. 


the  apex  of  the  styloid  process  to  the  angle  of  the  jaw;  the 
internal  lateral  from,  the  spine  of  the  sphenoid  to  the  mar- 
gins of  the  inferior  dental  foramen ;  the  external  lateral, 
which  alone  is  a  ligament  strictly  proper  to  the  articulation, 
reaches  from  the  outer  side  and  tubercle  of  the  zygoma  to 
the  outer  sm-face  of  the  neck  of  the  condyle. 

The  form  of  the  articulating  surfaces  and  the  compara- 
tive absence  of  retaining  ligaments  combine  to  allow  of  a 
variety  of  movement  unusual  in  any  other  than  a  ball  and 
socket  joint.  The  articulation  acts  as  a  simple  hinge  when 
the  jaw  is  simply  depressed,  and  this  is  the  only  motion 
possible  in  many  animals,  as  in  typical  carnivora.  When, 
however,  the  mouth  is  opened  to  the  fullest  possible  extent, 
the  condyle  leaves  the  glenoid  cavity,  slides  forward,  and 
rests  on  the  articular  eminence,  the  interarticular  fibro- 
cartilage  being  carried  forward  with  it.  The  passage  of  the 
condyle  on  to  the  articular  eminence,  although  always 
taking  place  when  the  lower  jaw  is  excessively  depressed, 
takes  place  sometimes  without  any  depression  of  the  lower 
jaw,  which  then  passes  horizontally  forward;  or  it  may  take 
place  on  the  one  side  only,  giving  to  the  jaw  the  lateral 
movement  so  useful  in  mastication.  In  the  mastication  of 
food  the  various  movements  are  combined,  or  succeed  one 
another  with  great  rapidity  ;  the  lateral  movements  are  not 
very  extensive,  the  outer  cusps  of  the  lower  teeth  of  one 
side  being  brought  to  antagonise  the  outer  cusps  of  the 
upper  teeth,  and  then  being  made  to  slide  forcibly  down  the 
sloping  surfaces  of  the  latter  till  they  return  to  their  normal 
antagonism ;  when  one  set  of  muscles  is  tired  the  same  pro- 
cess is  gone  through  on  the  other  side  of  the  mouth. 

The  closure  of  the  jaw,  and  the  rotatory  and  oblique 
motions,  are  accomplished  by  four  pairs  of  veiy  powerful 
muscles;  these  are  antagonised  by  muscles  comparatively 
feeble  ond  indirect  in  their  application. 

The  closure  of  the  jaws  is  effected  by  the  masseters  and 


34  A    MANUAL    OF   DENTAL    ANATOMY. 

the  temporals,  attached  to  the  outer  sides  of  the  jaw ;  and 
the  external  and  internal  pterygoids,  attached  to  its  inner 
sides. 

The  massetor,  temporal,  and  internal  pterygoid  muscles 
close  the  jaws  and  press  the  teeth  against  one  another,  and 

Fio.  15  (1). 


this  is  their  principal  action.  They  are  antagonised  by  the 
digastric,  the  mylohyoid,  and  the  geniohyoid  muscles,  which, 
aided  perhaps  hy  the  platysma,  depress  the  lower  jaw  when 
the  hyoid  bone  is  fixed  by  its  own  depressor  muscles. 

The  external  pterygoid  draws  the  jaw  forward,  and  so  in 
some  measure  tends  to  open  it ;  as  the  two  muscles  do  not 
always,  or  indeed  generally,  act  together,  they  give  a  lateral 
movement  to  the  jaw.  The  superficial  portions  of  the 
masseter  and  the  internal  pterygoid  are  ordinarily  supposed, 
as  their  direction  is  slightly  backwards,  to  assist  in  drawing 
the  jaw  forwai-ds,  but  Langer,  one  of  the  most  recent  inves- 
tigators of  their  action,  attaches  very  little  importance  to 
this,  and  indeed  considers  that,  when  the  jaw  has  been 
pulled  forwards  by  the  external  pterygoid,  the  combined 
action  of  the  internal  pterygoid,  the  temporal,  and  the  mas- 
seter, may  bring  it  back  again, 

(^)  Pterygoid  muscles.  1.  Upper,  and  2.  Lower  heads  of  external 
pterygoid  muscle.     3.  Internal  pterygoid  muscle. 


THE   MAXILLARY  BOXES.  35 

In  ordinary  mastication  the  various  movements  are  com- 
bined in  every  possible  manner. 

When  the  mouth  is  widely  open  the  condyles  play  upon 
the  articular  eminence  in  front  of  the  glenoid  cavity,  and 
the  external  pterygoid,  which  assists  in  widely  opening  the 
mouth,  draws  not  only  the  condyle,  but  also  the  inter- 
articular  fibro-cartilage  forwards,  so  that  the  latter  still 
intervenes  between  the  condyle  and  the  articular  eminence. 
The  interarticular  cartilages  do  not,  however,  accompany 
the  jaw  in  its  extreme  movement,  but  are  believed  only 
to  pass  forwards  as  far  as  that  part  of  the  eminence  which 
is  slightly  hollowed  out.  As,  however,  in  dislocation  they 
accompany  the  condyles,  this  supposition  may  be  incorrect. 

The  position  of  repose  is  neither  complete  closure  nor 
opening  of  the  jaws :  in  persons  with  enlarged  tonsils  the 
habitual  position  is  one  with  the  mouth  somewhat  more 
widely  open,  owing  to  the  difficulty  of  breathing  through  the 
nose;  a  fact  which  often  causes  an  irregularity  in  the 
disposition  of  the  teeth. 

The  axis  on  which  the  jaw  moves  is,  owing  to  the  bend 
of  the  ramus,  far  behind  the  glenoid  cavity ;  it  lies  veiy 
nearly  in  a  plane  formed  by  prolonging  the  plane  of  the 
masticating  sui-face  of  the  teeth. 

The  motions  executed  in  mastication  differ  much  according 
to  the  nature  of  the  food ;  hence  it  happens  that  in  different 
animals  the  muscles  of  mastication  are  very  variously 
developed. 

Thus,  in  the  Herbivora,  which  move  their  jaws  greatly 
from  side  to  side,  as  any  one  may  observe  for  himself,  the 
pterygoids,  and  especially  the  external  pterygoid,  attain  to 
a  very  large  relative  size. 

On  the  other  hand,  in  the  Rodents,  which  move  their 
jaws  backward  and  forwards  in  gnawing,  the  masseter  is 
enormously  developed,  and  has  a  very  marked  general 
backward  direction. 

D  2 


36 


A    MANUAL    OF  DENTAL    ANATOMY. 


Although  it  is  not  strictly  true,  the  masseter  and  temporal 
may  be  said  in  mammals  to  be  developed  in  an  inverse  ratio 
to  one  another  :  when  one  is  large  the  other  is  not. 

The  masseter  is  at  a  maximum  in  Caniivora,  which  have 


Fig.  1G  ('). 


little  lateral  movement  possible  to  their  jaws  ;  the  temporal 
is  also  highly  developed  in  many  of  the  class. 

In  the  great  apes,  the  temporal  becomes  enormously 
developed  only  at  the  period  of  second  dentition ;  this  fact, 
conjoined  with  its  size,  which  in  herbivora  seems  to  have  some 
relation  to  the  presence  or  absence  of  canines,  would  incline 
one  to  suppose  that  it  was  useful  in  that  rapid  closure  of  the 
mouth  appropriate  to  biting  when  animals  fight  or  seize  prey. 

The  form  of  the  glenoid  cavity  also  bears  an  intimate 
relation  to  the  dentition  of  the  animal,  and  the  nature  and 
extent  of  the  movement  of  its  jaws. 

Thus,  in  a  child  it  is  nearly  flat,  with  no  well  marked 
surrounding  elevations ;  its  axis  is  transverse,  and  little 
rotary  motion  is  made  use  of.      In  the  adult  it  is  deeply 


(*)  Condyle  of  the  lower  jaw,  and  glenoid  fossa  of  a  tiger. 


THE   MAXILLARY   BONES.  37 

sunk  :  the  axis  of  the  condyle  is  oblique,  and  rotary  move- 
ments are  largely  made  use  of  in  triturating  food. 

In  the  Felidie,  it  is  strictly  transverse ;  their  teeth, 
adapted  for  slicing  but  not  grinding,  would  gain  nothing 
by  lateral  motion,  vs^hich  is  rendered  quite  impossible  by 
the  manner  in  which  the  long  transverse  condyles  are 
locked  into  the  glenoid  cavity  by  strong  processes  in  front 
and  behind.  Curiously  enough  the  interarticular  cartilage 
is  present,  but  as  the  condyle  never  moves  forward,  the 
cartilage  is  not  attached  to  the  external  pterygoid  muscle. 

In  Herbivora  the  condyle  is  roundish,  the  ascending  ramus 
long,  the  pterygoid  muscles  large,  and  the  glenoid  cavity 
shallow ;  in  the  whale,  which  of  course  does  not  masticate  at 
all,  there  is  no  interarticular  cartilage,  and  no  synovial  mem- 
brane ;  the  articulation  is  reduced  to  a  mere  ligamentous 
attachment. 

The  harder  a  substance  is,  the  farther  back  between  the 
molars .  it  is  placed ;  and  as  the  food  escapes  from  between 
the  teeth  it  is  constantly  being  replaced  by  the  lips,  cheeks, 
and  tongue,  the  buccinator  muscle  being  largely  concerned 
in  this  work  of  preventing  morsels  of  food  from  escaping 
from  the  teeth  during  its  mastication. 

Just  as  the  muscles  of  mastication  vary  in  their  relative 
development  in  accordance  with  the  food  to  be  dealt  with, 
so  also  do  the  salivary  glands. 

As  a  rule  herbivorous  creatures  have  large  parotid  glands; 
that  is  to  say,  those  creatures  which  deal  with  the  driest 
food  and  masticate  it  the  most  have  this  gland  largely 
developed.  For  instance  it  is  very  large  in  Ruminants ; 
in  Herbivorous  Marsupials  it  is  larger,  in  the  carnivorous 
section  smaller,  than  the  submaxillaries.  When  an  espe- 
cially viscid  fluid  is  required,  as,  for  example,  that  which 
lubricates  the  tongue  of  an  ant-eater,  this  is  furnished 'by 
exceedingly  large  submaxillary  glands. 

The  nerves  of  the  teeth  are  derived  from  branches  of  the 


38  A    MANUAL    OF  DENTAL    ANATOMY, 

fifth  nerve,  the  nerve  of  sensation  of  the  whole  side  of  the 
face  and  head  :  the  lower  teeth  through  the  inferior  maxil- 
lary nerve,  the  upper  through  the  anterior  and  posterior 
dental  branches  of  the  superior  maxillary  nerve.  The 
nerves  are  given  off  from  the  nerve  trunks  in  bundles 
corresponding  in  number  to  the  roots  of  the  teeth  for 
which  they  are  destined.  For  the  details  of  the  distribu- 
tion of  the  fifth  nerve  the  student  must  refer  to  works 
treating  of  anatomy,  as  it  would  be  out  of  place  to  enter 
upon  the  subject  at  length  in  these  pages,  in  which  merely 
one  or  two  matters  of  special  interest  to  the  dental  student 
will  be  touched  upon. 

In  the  case  of  the  inferior  maxillary  nerve  the  roots  of 
the  teeth  come  into  very  close  proximity  with  the  main 
trunk  of  the  nerve ;  this  is  especially  the  case  with  the 
lower  wisdom  teeth.  Within  a  few  days  of  writing  these 
lines  I  extracted  a  lower  wisdom  tooth  (with  forceps)  for 
a  gentleman,  who,  immediately  after  the  extraction,  in- 
quired if  he  could  have  bitten  his  lip,  as  it  felt  swollen; 
on  testing  it  I  found  slight  but  well  marked  numbness  on 
that  side  of  the  lip  and  chin,  which  did  not  wholly  subside 
before  he  left  me.  In  this  case  a  groove  upon  the  under 
surface  of  the  much  curved  roots  appeared  to  indicate  that 
the  nerve  trunk  was  in  close  contact  with  the  tooth. 

No  reason  is  at  present  known  why  the  tooth  pulp  should 
be  so  richly  supplied  with  nerves,  as  no  obvious  advantage 
results  therefrom.  Teeth  with  persistent  pulps  which  go  on 
growing  throughout  the  life  of  the  animal,  have  always  large 
nerves  :  thus  a  very  large  trunk  goes  to  the  pulp  of  a  rodent 
incisor.  But  although  in  this  case  the  rich  nervous  supply 
doubtless  has  to  do  with  nutrition,  and  presides  over  the 
great  formative  activity  of  the  tissue,  this  does  not  fully 
account  for  the  pulps  of  the  teeth  of  limited  growth  being  so 
amply  supplied  with  nerves. 

As  has  been  mentioned  in  the  description  of  the  lower 


Fic.  17  ('). 


(1)  KiAi'^UAM  nj.  Tin:  DisTKiBriios  iij    irii:  }VnANrin>  oi   tiii;  KifTii  NiRvr. 
fFrnni  Torac-,'  ';  Lectures  on  Dental  Dijsiiili.gy  iuid  f^uigrry  "— dian-n  by  Mr.  C.  De  Mnrgan,) 
Orhthalinlc  division  :— 1.  Frontal.    2.  Nasal  and  long  ciliary.    3.  Branches  to  ciliary  ganglion. 
Superior  maxillary  division  :— 1.  Orbital  |  Jnl,"^',"*'''^   }      •"''.  Sphenopalatine   (WckrrsT)  ganglion. 

IV  rostrrlnr  drufcil,  iiasslnu  down.    7,  S.  Anterior  d.-ntal.    -.i.  Int'ra-orhital. 
Inferior  niiixlllary  divlsron  :- 10.  Aurieulo-tcmporHl.     11,  Alasseteric.     VJ.   Deep  tcMiporal.     ix    Pteiv. 
-Old.    14.  Buccalto  bucdnator,  &c.    10.  Gustatory,    l(j.  Jlylohyoid  branch.    17.  Inferior  dent -il 
IS.  Mental. 

[To/m-cp.dS. 


THE   MAXILLARY   NERVES.  39 

maxillary  bone,  the  inferior  dental  nerve  emerges  from  the 
bone  by  the  mental  foramen,  near  to  the  end  of  the  roots  of 
the  bicuspid  teeth.  Pain  due  to  distant  causes  is  often 
refei'red  to  the  point  of  emergence  of  a  nerve,  as  is  so 
frequently  exemplified  in  supraorbital  neuralgia;  in  the 
same  way  pain  due  to  diseased  teeth  far  back  in  the  lower 
jaw  (especially  to  wisdom  teeth),  is  frequently  i-efen-ed  to 
the  bicuspid  region.  Curiously  enough,  though  there  is  no 
apparent  close  parallel  in  the  disposition  of  the  nerves,  a 
similar  reference  of  pain  to  the  bicuspid  region  is  occasionally 
observed  in  the  upper  jaw.  And  it  may  be  added  that  there 
is  very  probably  some  closer  parallel  in  the  minute  disposi- 
tion of  the  nerve  fibres  going  to  the  teeth  in  the  uj)per  and 
lower  jaws  than  is  recognisable  by  rough  anatomical  pro- 
cesses, for  while,  to  all  appearance,  the  nerve  trunks  are 
differently  arranged,  it  is  a  matter  of  almost  everyday  ob- 
servation to  find  pain  due  to  one  tooth  referred  with  pre- 
cision to  its  fellow  in  the  other  jaw. 

The  lower  teeth  derive  their  vascular  supply  from  the 
branches  given  off  to  each  tooth  by  the  inferior  dental  artery, 
itself  a  branch  of  the  internal  maxillary ;  the  upper  teeth 
derive  their  ai'tex'ies  from  the  superior  dental,  a  part  of 
the  alveolar  branch  of  the  internal  maxillary,  which  supplies 
the  molar  and  bicuspid  teeth ;  and  the  front  teeth  from  the 
descending  branch  of  the  infraorbital,  the  vessels  thus  having 
an  arrangement  somewhat  analogous  to  that  of  the  nerves. 

The  distribution  of  the  veins  corresponds  closely  to  that 
of  the  arteries. 

No  lymphatics  have  been  traced  into  the  teeth. 


Tomes,  J.     Lectures  on  Dental  Physiolog-y  and  Surgery.     1848. 
HA.ERIS0N  Allek.     Anatomy  of  the  Facial  Region,  Dental  Cosmos, 

1874. 
Cattlin.     Anatomy   of   Antram.     Trans.    Odontological   Society, 
1857. 


CHAPTER  III. 

THE    DENTAL    TISSUES. 

It  is  usual  to  speak  of  there  being  two  kinds  of  teeth, 
namely,  horny  or  albuminous,  and  calcified  teeth ;  but  of  the 
development  of  the  former  nothing  is  accurately  known,  and 
it  is  hence  impossible  to  determine  in  what  relation  they 
really  stand  to  other,  or  calcified,  teeth. 

These  latter  are  composed  of  one  or  more  structures, 
which  are  in  great  measure  peculiar  to  the  teeth  (although, 
what  is  to  all  intents  and  purposes  dentine,  is  to  be  found 
in  the  skeletons  and  in  the  dermal  appendages  of  some  fish, 
and  other  exceptions  might  be  found  to  the  absolute  accu- 
racy of  the  statement),  and  hence  are  called  "  dental  tissues." 
Notwithstanding  the  existence  of  certain  transitional  forms, 
it  is  not  possible  to  doubt  the  propriety  of  a  general  division 
of  dental  tissues  into  three,  viz.,  Dentine,  Enamel,  and 
Cementum. 

The  first  named  of  these  constitutes  the  greater  part  of 
all  teeth,  and  so  far  predominates  in  mass  over  the  other 
constituents  that,  in  very  many  cases,  the  tooth  would 
retain  its  form  and  character  after  the  removal  of  the  enamel 
and  cementum. 

This  central  body  of  dentine,  enclosing  the  pulp,  is  very 
often  covered  by  a  cap  of  enamel,  which  forms  the  surface 
of  the  tooth  ;  this  may  be  very  partial,  as  in  the  eel  or  the 
newt,  in  which  animal  only  this  enamel-capped  tip  of  the 
tooth  projects  far  above  the  surface  of  the  mucous  membrane  ; 
or  it  may  cover  a  much  larger  proportion  of  the  tooth,  as 


THE   DENTAL    TISSUES. 


in  man.  Perhaps  the  most  usual  condition  is  that  the 
enamel  invests  the  whole  crown  of  the  tooth,  stopping  short 
at  about  the  level  to  which  the  gum  reaches,  as  in  the 
human  and  most  other  mammalian  teeth  of  limited  growth. 
In  teeth  of  persistent  growth  the  enamel  extends  down  into 
the  socket  as  far  as  the  base  of  the  tooth ;  in  such  cases  it 
may  embrace  the  whole  cu'cumference  of  the  dentine,  as  in 
the  molar  teeth  of  many  rodents,  or  it  may  be  confined  to 
one  side  only,  as  in  their  incisor  teeth,  where  by  its  greater 
hardness  it  serves  to  constantly  preserve  a  sharp  edge  as 
the  tooth  is  worn  away.  The  enamel  is  believed  to  be  quite 
absent  from  many  teeth  ;  thus  the  subclass  Edentata  compris- 
ing sloths,  armadillos,  and  ant-eaters  have  it  not ;  the  narwal, 
certain  cetaceans,  some  reptiles,  and  many  fish  have  none. 

But  although  it  might  appear  an  exceedingly  simple 
matter  to  determine  whether  a  tooth  is  or  is  not  coated  with 
enamel,  as  a  matter  of  fact  in  practice  it  is  not  always  easy 
to  be  certain  upon  this  point.  When  the  enamel  is  tolerably 
thick  there  is  no  difiiculty  in  making  sections  which  show 
it  satisfactorily,  but  when  it  is  very  thin  it  is  apt  to 
break  otf  in  grinding  down  the  section.  And  even  when  it 
does  not,  it  is  in  such  cases  usually  quite  transparent  and 
structureless,  and  the  outermost  layer  of  the  dentine  being 
also  clear  and  structureless,  it  is  very  hard  to  decide  whether 
the  appearance  of  a  double  boundary  hne  is  a  mere  optical 
eflfect  due  to  the  thickness  of  the  section,  or  is  indicative  of 
a  thin  layer  of  a  distinct  tissue  which  might  be  either 
enamel  or  cementum. 

My  own  investigations  upon  the  development  of  the  teeth 
of  fish  and  reptiles  have  led  me  to  suspect  that  rudimentary 
layers  of  enamel  exist  upon  many  teeth  on  which  their 
presence  has  not  been  recognised,  for  I  have  found  that  the 
formative  enamel  organs  occur  universally.  Upon  the  teeth 
of  snakes,  which  were  stated  by  Professor  Owen  to  be  com- 
posed only  of  dentine  and  cement,  I  have  endeavoured  to 


42  A    MANUAL    OF   DENTAL    ANATOMY. 

show  that  a  thin  layer  of  enamel  exists,  and  that  there  is  no 
cementum.  The  frog  has  an  enamel  organ  as  distinct  as 
that  of  the  snake,  but  I  am  hardly  positive  that  there  is 
enamel  upon  its  teeth,  although  there  is  an  appearance  of  a 
thin  coat  of  distinct  tissue.  I  have  also  demonstrated  that 
the  armadillo  has  an  enamel  organ,  but  have  failed  to  discover 
any  enamel  or  anything  like  it  upon  its  teeth,  and  Professor 
Turner  has  made  a  similar  observation  upon  the  narwal. 

At  all  events  we  may  safely  say  that  in  these  and  many 
other  creatures  no  functional  development  of  enamel  takes 
place  :  whether  it  does  or  does  not  exist  in  an  extremely 
thin  and  rudimentary  layer  has  become  a  question  of  much 
less  significance,  since  I  have  shown  the  presence  of  an 
enamel  organ  to  be  universal  at  an  early  stage. 

Hence  I  feel  some  hesitation  in  endorsing  Professor  Owen's 
generalisation  that  the  dentine  is  the  most  and  enamel  the 
least  constant  of  dental  tissues ;  it  is  possible  that  it  may  be 
so,  but  recent  researches  into  the  development  of  teeth  have 
very  materially  modified  the  conceptions  formed  as  to  the 
relations  of  the  dental  tissues  to  one  another,  and  must  lead 
us  to  examine  carefully  into  such  deductive  statements 
before  accepting  them. 

The  remaining  dental  tissue  is  cementum,  which  clothes, 
in  a  layer  of  appreciable  thickness,  the  roots  of  the  teeth, 
and  reaches  up  as  far  as  the  enamel,  the  edge  of  which  it 
overlaps  to  a  slight  extent ;  when  the  cementum  is  present 
upon  the  crown,  it  occupies  a  position  external  to  that  of 
the  enamel.  Cementum  occurs  universally  upon  the  teeth 
of  mammalia,  but  it  is  not  always  confined  to  the  root  of 
the  tooth  ;  in  many  teeth  of  persistent  growth  it  originally 
invested  the  whole  crown,  and  after  it  has  been  worn  from 
the  exposed  grinding  surface,  continues  to  invest  the  sides 
of  the  tooth.  (See  the  description  of  the  complex  teeth  of 
the  elephant,  cow,  horse,  &c.) 

It  is  probably  entirely  absent  from  the  teeth  of  snakes,  and 


THE   DENTAL    TISSUES.  43 

indeed  of  very  many  reptiles ;  in  the  reptilian  class,  at  all 
events,  it  appears  to  me  to  be  confined  to  those  in  which  the 
teeth  are  lodged  either  in  sockets  or  in  a  deep  bony  groove, 
as  I  am  unacquainted  with  any  tooth  anchylosed  to  the  jaw 
in  which  it  exists,  imless  we  are  inclined  to  include  under  the 
term  cementum  the  tissue  which  I  have  designated  "  bone 
of  attachment,"     (See  "  Implantation  of  Teeth.") 


Upon  the  outer  surface  of  the  dentine  the  enamel  forms 
a  cap  of  a  very  much  harder  and  denser  material.  In 
its  most  perfect  forms  it  is  very  far  the  hardest  of  all 
tissues  met  with  in  the  animal  body,  and  at  the  same 
time  the  poorest  in  organic  matter.  In  the  enamel  of  a 
human  adult  tooth  there  is  as  little  as  3|  to  5  per  cent, 
of  organic  matter,  and,  judging  from  its  brittleness  and 
transparency,  there  is  probably  even  less  in  the  enamel  of 
some  lower  animals ;  the  lime  salts  consist  of  a  large  quan- 
tity of  phosphate,  some  carbonate,  and  a  trace  of  fluoride 
of  calcium ;  in  addition,  there  is  a  Httle  phosphate  of 
magnesium. 

Von  Bibra  gives  two  analyses  of  enamel  : 


Calciiun  Phosphate  and  Fluoride 
Calcium  Carbonate  . 
Magnesium  Phosphate 
Other  Salts       .... 
Cartilage      .... 
Fat 

Organic        .... 
Inorganic  .... 

The  cap  of  enamel  is  of  varying  thickness,  being  thicker 
in  the  neighbourhood  of  cusps  than  elsewhere ;   in   teeth 


ADULT 

ADULT 

MAN. 

WOMAN. 

89-82 

81-63 

4-37 

8-88 

1-34 

2-55 

•88 

•97 

3-39 

5-97 

•20 

a  trace 

3-59 

5-97 

9G-41 

94-03 

A    MANUAL    OF   DENTAL    ANATOMY. 


of  limited  growth  it  terminates  by  a  thin  edge  at  the 
neck  of  the  tooth,  where  it  is  overlapped  to  some  slight 
extent  by  the  cementum.  When  a  thick  coating  of  cemeu- 
tum  exists  over  the  whole  crown,  this  lies  outside  the 
enamel,  the  proper  jjlace  of  Avhich  is  therefore  between  the 
cementum  and  the  dentine. 

The  external  surface  of  the  enamel  is  finely  striated,  the 
course  of  the  strife  being  transverse  to  the  long  axis  of  the 
crown  ;  in  addition  to  this  very  fine  striation,  there  may  be 
a  few  deeper  and  more  pronounced  grooves  or  pits,  which 
are  pathological,  and  are  marks  of  checks  in  development 
more  or  less  complete.  The  enamel  of  some  animals  is,  to 
all  appearance,  structureless ;  such  is  the  nature  of  the 
little  caps  which,  like  spear  points,  surmount  the  teeth  of 
fishes  of  the  eel  tribe,  cod  tribe,  or  of  the  newt,  and  which 
from  their  extreme  brittleness  are  often  lost  in  preparing 
sections,  so  that  their  very  existence  has  long  been  over- 
looked. But  the  absence  of  structure,  if  such  it  really  be,  is 
after  all  a  mere  question  of  degree  ;  in  the  commonest  form 
of  enamel,  such  as  that  of  the  human  teeth,  there  is  a  finely 
fibrous  structure,  very  apparent  in  imperfect  teeth,  but  far 
less  so  in  well-formed  ones,  and  the  enamel  of  the  eel  is,  in 
the  manner  of  its  development,  fibrous  ;  so  that  even  though 
we  cannot  distinguish  its  constituent  fibres  when  it  is  com- 
pleted, this  is  merely  an  indication  that  calcification  has 
progressed  a  little  farther  than  in  human  teeth  :  if  calcifica- 
tion only  goes  far  enough,  all  structure,  if  not  destroyed, 
will  at  all  events  be  masked  from  sight. 

The  structure  of  hviman  enamel  has  been  stated  to  be 
fibrous ;  that  is  to  say,  it  has  a  cleavage  in  a  definite  direc- 
tion, and  is  capable  of  being  broken  up  into  fibres  or  prisms, 
which  seem  in  transverse  section  to  approximate  more  or 
less  closely  to  hexagonal  forms  brought  about  by  their 
mutual  apposition.  The  general  direction  pursued  by  the 
prisms  is  one  from  the  dentine  towards  the  surface  ;  this  is. 


THE   DENTAL    TISSUES. 


45 


however,  subject  to  many  minor  modifications.  The  curved 
and  decussating  course  of  the  human  enamel  prisms  renders 
them  difficult  to  trace  throughout  their  length,  but  the 
structure  of  the  enamel  of  many  lower  animals  (especially 
the  rodents)  is  more  easily  intelligible.  Enamel  such  as 
that  of  the  ^Manatee,  in  which  all  the  prisms  pursue  a 
perfectly  straight  course,  is  of  comparatively  rare  occurrence, 
but  among  the  rodents  the  com-ses  pursued  by  the  enamel 
prisms  are  simple,  and  produce  very  regular  patterns,  which 
are  constant  for  particular  families  (J.  Tomes).  Thus,  in 
the  Sciuridce,  a  section  of  the  enamel,  whether  longitudinal 


Fig.  is  (i). 


or  transverse,  appears  divided  into  an  outer  and  inner 
portion,  in  which  the  prisms,  although  contimious  from  the 
dentine  to  the  free  surface,  piu'sue  a  different  direction.  As 
seen  in  the  longitudinal  section,  the  enamel  prisms  start 
from  the  dentine  at  right  angles  to  its  surface,  and  after 
passing  through  about  two-thirds  of  the  thickness  of  the 
enamel  in  this  direction,  abruptly  bend  upwards  at  an  angle 
of  45  degi'ees  with  their  original  course.  In  transverse 
section  the  enamel  prisms  are  found  to  be  arranged  in  hori- 

(')  Section  of  dentine  and  enamel  of  a  Beaver  :  in  the  inner  half  the 
prismfi  of  contiguous  layei's  cross  each  other  at  right  angles,  iu  the  outer 
they  are  parallel. 


46  A    MANUAL    OF   DENTAL    ANATOMY. 

zontal  layers,  each  layer  being  a  single  fibre  in  thickness  ; 
in  alternate  layers  the  prisms  pass  to  the  right  and  to  the 
left,  crossing  those  of  the  next  layer  at  right  angles,  and 
thus  making  a  pattern  of  squares  in  the  inner  two-thirds  of 
the  enamel.  But  in  the  outer  third  of  the  enamel,  ■where 
the  prisms  bend  abruptly  upwards,  those  of  superimposed 
layers  no  longer  pass  in  opposite  directions,  but  are  all 
parallel ;  in  fact  no  longer  admit  of  distinction  into  laminee. 

Thus  each  enamel  prism  passes  in  a  veiy  definite  direction 
and,  seen  with  those  of  other  layers,  forms  a  very  charac- 
teristic pattern  ;  but  the  enamel  prisms  are  not  in  any  part 
of  their  course  curved. 

In  the  beaver,  from  which  the  foregoing  figure  is  taken, 
the  an-angement  of  the  enamel  prisms  is  dissimilar  in  the 
upper  and  lower  teeth,  the  lamination  taking  place  in 
difierent  directions,  so  that  a  longitudinal  section  of  the  one 
might,  so  far  as  this  is  concerned,  be  mistaken  for  a  trans- 
verse section  of  the  other.  As  regards  the  decussation  of 
the  prisms  of  alternate  layers,  it  is  similar  to  that  of  the 
Sciuridce,  but  it  difi^ers  in  the  laminae  being  slightly  flexuous 
instead  of  pursuing  perfectly  straight  lines. 

Among  the  porcupine  family  very  much  more  complex 
patterns  are  met  with,  the  enamel  prisms  being  individually 
flexuous,  and  their  curves  not  being  confined  to  one  plane  ; 
the  individual  prisms  pursue  a  serpentine  coiurse,  and  cannot 
be  followed  far  in  any  one  section.  Near  to  the  surface, 
however,  they  all  become  parallel,  the  enamel  thus  conform- 
ing with  that  of  other  rodents  in  being  divided  into  two 
portions  (at  least  so  far  as  the  course  pursued  by,  and  the 
pattern  traced  by,  its  fibres  in  its  inner  and  outer  parts  can 
be  said  to  so  divide  it).  The  Leporidce,  or  hares,  form  an 
exception  to  this  ;  their  enamel  has  no  such  lamelliform 
arrangement,  but  is  built  up  merely  of  slightly  flexuous 
prisms. 

By  tracing  the  coiirses  of  enamel  prisms  from  the  simple 


THE   DENTAL    TISSUES.  47 

pattern  found  in  tlie  Manatee  through  that  of  the  squirrel 
and  dormouse,  and  the  porcupine,  we  see  how  a  very  definite 
arrangement,  at  first  simple,  becomes  modified  into  some- 
thing a  little  more  complex,  till  at  last  it  reaches  a  degree 
of  complexity  that  looks  like  mere  disorder.  No  one  un- 
familiar with  the  enamel  of  other  rodents,  looking  at  the 
enamel  of  the  porcupine,  Avould  be  able  to  unravel  the  very 
indefinite  looking  chaos  of  prisms  before  him ;  but  had  he 
studied  forms  in  some  degree  transitional  he  could  not  doubt 
that  the  tortuous,  curving  course  which  he  saw  the  prism 
to  be  pursuing  was  nevertheless  perfectly  definite  and  precise, 
and  formed  part  of  a  regular  pattern. 

In  perfectly  healthy  human  enamel  the  fibrillar  arrange- 
ment is  not  so  very  strongly  marked ;  the  prisms  ai'e  solid, 

Fig.  19  (i). 


are  apparently  in  absolute  contact  with  one  another,  without 
visible  intervening  substance. 

But  Bodecker,  basing  his  conclusions  upon  the  examination 
of  thin  sections,  stained  with  chloride  of  gold,  holds  that 
enamel  is  built  up  of  columns  of  calcified  substance,  between 
which  minute  spaces  exist.     These  are  filled  by  a  material 

(')  Human  enamel,  from  the  masticating  surface  of  a  molar.  The 
figure  is  merely  intended  to  show  the  general  direction  of  the  fibres. 


48  A    MANUAL    OF   DENTAL    ANATOMY. 


Avhich  takes  the  stain  deeply,  and  is  probably  analogous  to 
the  cement  substance  of  epithelial  foi-mations.  As  seen  in 
sections,  it  gives  off  exceeding  fine  thorns,  which  apparently 
pierce  the  prisms  at  right  angles  to  their  length,  so  that  it 
forms  a  close  network  very  intimately  mixed  up  with  the 
calcified  portion  of  the  enamel. 

It  is  not  of  uniform  thickness,  but  is  beaded,  and  BiJdecker 
attributes  to  it  a  role  of  far  greater  importance  than  that  of 
a  mere  cementing  substance,  for  he  regards  it  as  being  an 
active,  protoplasmic  network,  which  renders  the  enamel 
much  more  "  alive  "  than  it  has  hitherto  been  considered  to 
be.  He  believes  it  to  become  continuous  with  the  soft 
contents  of  the  dentinal  tubes  through  the  medium  of  large 
masses  of  protoplasmic  matter  found  at  the  margins  of  the 
enamel  and  dentine. 

But  although  there  are  various  reasons  for  suspecting  that 
enamel  is  not  completely  out  of  the  pale  of  nutrition  from 
the  moment  that  a  tooth  is  cut,  yet  further  observations  are 
needed  before  the  activity  and  importance  of  the  cement 
substance  demonstrated  by  Bodecker  can  be  held  to  be  fully 
established.  Klein  remarks  that  "  the  enamel  cells,  like  all 
epithelial  cells,  being  separated  from  one  another  by  a  homo- 
geneous interstitial  substance,  it  is  clear  that  the  remains  of 
this  substance  must  occur  also  between  the  enamel  prisms  ; 
in  the  enamel  of  a  developing  tooth  the  interstitial  substance 
is  larger  in  amount  than  in  the  fully  formed  organ.  It  is 
improbable  that  nucleated  protoplasmic  masses  are  con- 
tained in  the  interstitial  substance  of  the  enamel  of  a  fully- 
formed  tooth,  as  is  maintained  quite  recently  by  Bodecker." 
The  study  of  the  development  of  marsupial  enamel,  to  be 
alluded  to  at  a  future  page,  by  showing  that  the  enamel  is 
penetrated  by  soft  tissue  differently  placed,  also  tends  to 
throw  doubt  upon  Bodecker's  interpretation.  W.  J.  Barkas 
{3Ionthhj  Review  of  Dental  Surgery,  1874)  has  also  perhaps 
had  imder  observation  this  cementing  substance  ;    he  also 


THE   DENTAL    TISSUES. 


believes  that  the  enamel   prisms  are  tubular,  minute  canals 
running  along  their  axes. 

On  the  whole  the  prisms  are  parallel,  and  run  from  the  sur- 
face of  the  dentine  continuously  to  that  of  the  enamel.  Their 
paths  are  not,  however,  either  perfectly  straight  or  perfectly 
parallel,  for  alternate  layei's  appear  to  be  inclined  in  opposite 
directions,  while  they  are  also  wavy,  forming  several  curves 
in  their  length.  The  curvature  of  the  enamel  prisms  is  most 
marked  upon  the  masticating  surface ;  while  the  layers, 
alternating  in  the  direction  of  their  inclination  as  just 
described,  are  in  planes  transverse  to  the  long  axis  of  the 
cro^\^l,  and  correspond  to  the  fine  striee  on  the  surface  of 
the  enamel,  which  appear  to  be  caused  by  their  outcrop. 
The  curvatures  take  place  in  more  than  one  plane ;  in  other 
words,  the  course  of  the  individual  prism  is  more  or  less  of 
a  spiral. 

Although  most  prisms  run  through  the  whole  thickness  of 
the  enamel,  yet  inasmuch  as  the  area  of  the  outer  is  much 
larger  than  that  of  the  inner  surface  of  the  enamel,  and  the 
individual  prisms  do  not  u.ndergo  any  alteration  in  size  as 
they  pass  outwards,  many  supplemental  fibres  are  present  in 
the  outer  portions  of  the  enamel  which  do  not  penetrate  far 
inwards. 

The  individual  fibres  are  to  all  appeai*ance  structureless 
in  perfectly  formed  human  enamel,  but  a  faint  transveree 
striation,  fainter,  but  otherwise  not  unlike  that  of  voluntary 
muscle,  is  so  general  that  it  cannot  be  regarded  as  patho- 
logical, although  it  is  most  strongly  developed  in  imperfect 
brownish  enamel.  The  striation  in  question  may  be  seen 
even  in  a  single  isolated  fibre,  and  is  not  necessarily  con- 
tinuous over  adjacent  fibres,  though  it  often  is  so  ;  it  is 
rendered  more  apparent  by  the  slight  action  of  diluted  acids 
upon  the  fibre.  Very  various  interpretations  of  this  appear- 
ance have  been  given.  It  has  been  attributed  to  "  an  inter- 
mittent calcification"  of  the  enamel  fibi-e  (Hertz),   but  is 


50  A    3fANUAL    OF  DENTAL    ANATOMY. 


with  more  probability  referred  to  varicosities  in  the  indi- 
vidual fibres  (Kulliker,  Waldeyer)  (').  It  is  very  marked  in 
the  enamel  of  the  common  rat,  which  shares  with  that  of 
other  mnridte  the  peculiarity  of  having  the  individual  fibres 
almost  serrated,  those  of  adjacent  crossing  layers  being  fitted 
to  one  another  Avith  great  exactness.  In  human  enamel  the 
adjacent  fibres  if  united  without  any  intermediate  cement- 
ing medium,  and  pursuing  courses  slightly  different,  must 
of  necessity  be  of  slightly  irregular  form,  or  else  interspaces 
would  be  left,  which  is  not  found  to  bo  the  case.  Thus  the 
"decussation  of  the  fibres"  is  a  plausible  explanation  of  this 
appearance  of  striation ;  indeed  isolated  filires  do  present  an 
appearance  of  slight  varicosities,  repeated  at  regular  in- 
tervals. The  penetration,  at  regular  inten^als,  of  the  prisms 
by  the  "  thorns  "  of  cement  substance  (sec  page  48),  affords 
another  explanation. 

Although  the  perfect  enamel  fibre  appenrs  to  be  entirely 

Fig.  20  (-). 


a    -.. 


homogeneous,  it  is  not  really  so,  for  acids  act  with  far  greater 
rapidity  upon  the  central  or  axial  portion  of  the  fibi-e  than 
upon  its  periphery.     The  accompanying  figujre,  taken  from 

(^)  The  striation  of  voluntary  muscle  has  been  very  recently  proved  to 
he  due  to  this  same  cause  (Dr.  Haycraft,  "  Proceedings  of  Royal  Society," 
Feb.  1881). 

(")  From  human  enamel,  softened  in  chromic  acid,  until  it  coi^ld  be 
j'eadily  cut  with  a  knife. 


THE   DENTAL    TISSUES.  51 


enamel  softened  by  prolonged  maceration  in  a  1  per  cent, 
solution  of  chromic  acid  shows  this  well ;  the  central  por- 
tions of  the  fibre  are  dark,  and  are  stained  green  by  the 
rednced  chromium  sesquioxide,  while  the  clear  interspaces 
are  colourless.  Again,  if  dilute  hydrochloric  acid  be  applied 
to  a  section  of  enamel,  the  axial  parts  of  the  fibres  are  first 
attacked  and  are  dissolved  away,  so  that,  if  the  section  be 
transverse,  a  fenestrated  mass  remains.  During  the  forma- 
tion of  enamel  the  hardening  salts  are  deposited  first  in  the 
periphery  of  the  enamel  cells,  so  that  the  youngest  layer  of 
enamel  is  full  of  holes,  each  one  of  which  corresponds  to  the 

Fig.  21  ('). 


centre  of  a  fibre.  Although  calcification  goes  on  to  obliterate 
the  visible  difference  between  the  centre  and  the  periphery 
of  the  enamel  fibre,  yet  the  action  of  an  acid  reverses  the 
order  of  its  formation  and  once  more  makes  it  fenestrated, 
indicating  that  there  is  not  absolute  identity  of  substance 
in  the  inner  part  of  the  fibre.  In  imperfect  enamel,  indeed, 
a  central  narrow  canal  has  sometimes  been  observed  in  the 
interior  of  an  enamel  fibre. 

In  fractured  enamel,  the  line  of  fracture  is  said  to  run 
through  the  centre  of  the  fibres,  and  not,  as  might  have  been 
expected,  through  their  interspaces. 

There  is  also  an  appearance  of  striation  upon  a  far  larger 
scale,  consisting  of  brownish  lines,  which  are  never,  or  almost 
never,  quite  parallel  with  the  outer  surface  of  the  enamel, 
but  which  nevertheless  preserve  some  sort  of  parallelism 
with  it  and  the  surface  of  the  dentine.     These  are  known 

(1)  Transverse  section  of  enamel,  the  axial  portion  of  the  jirisms  ha\dng 
been  removed  by  dilute  hj'droohloric  acid. 

E  2 


A    MANUAL    OF   DENTAL    ANATOMY. 


as  the  "  brown  strife  of  Retzius,"  and  as  they  coincide  with 
what  was  at  one  time  the  outer  surface  of  the  enamel  cusp, 

Fig.  22  ('). 


are  in  some  sense  marks  of  its  stratification,  in  its  original 
deposition. 

Pigment  is  seen  in  the  enamel  of  many  rodents  ;  it  is  in 

(')  Cavities  in  human  enamel,  whicli  communicate  with  the  dentinal 
tubes. 


THE   DENTAL    TISSUES.  53 

the  outer  layers  of  the  enamel,  but  has  no  sharply  defined 
boundaiy,  fading  away  gi-adually  into  the  colourless  tissue 
lying  within  it.  Some  authors  have  supposed  that  the  pig- 
ment lay  in  a  thin  coating  of  cementum,  or  in  a  very  dis- 
tinct layer  of  enamel,  but  as  has  above  been  stated,  such  is 
not  the  case. 

Cavities  of  irregular  form  sometimes  exist  in  the  enamel 
near  to  the  surface  of  the  dentine,  and  when  such  spaces 
exist  the  dentinal  tiibes  sometimes  communicate  with  them, 
but  these  are  perhaps  to  be  regarded  as  pathological ; 
Bodecker  regards  them  as  filled  up  l)y  protoplasm.  Irregular 
fissures  and  cavities  also  occur  upon  the  outer  surface  of  the 
enamel,  which  also  have  no  special  significance  save  as  pre- 
disposing causes  of  dental  caries. 

In  man,  however,  dentinal  tubes  may  occasionally  be  seen 
to  enter  the  enamel,  passing  across  the  boundary  between 
the  two  tissues,  and  pursuing  their  course  without  being  lost 
in  irregular  cavities ;  though  this  appearance  is  seldom  to  be 
found.  As  was  pointed  out  by  my  father,  the  passage  of  the 
dentinal  tubes  into  and  through  a  great  part  of  the  thickness 
of  the  enamel  takes  place  in  marsupials  with  such  constancy 
as  to  be  almost  a  class  characteristic. 

The  only  exception  to  the  rule  amongst  recent  marsupials 
occiu-s  in  the  wombat,  in  which  no  dentinal  tubes  enter  the 
enamel ;  those  extinct  marsupials  which  have  been  examined 
present,  as  might  have  been  expected,  a  structure  in  this 
respect  similar  to  that  of  their  nearest  allies  amongst  the 
recent  genera. 

The  enamel  of  the  wombat  is  peculiar  also  in  another 
respect,  being  covered  by  a  strong  and  remarkably  uniform 
layer  of  cementum. 

The  penetration  of  the  enamel  l)y  dentinal  tubes  is  not, 
however,  a  peculiarity  quite  confined  to  the  marsupials,  for 
it  is  to  be  found  in  some  rodents  (e.ff.  the  jerboa),  and  in 
some  insectivoia  (e.r/.,  the  soricida)). 


5i 


A    MANUAL    OF  DENTAL    ANATOMY. 


Fia.  23  (•). 


^1     "ill 


(1)  Enamel  and  dentine  of  a  Kangaroo  {Macropiis  major). 
The  dentinal  tubes  in  the  dentine  (A)  are  furnished  with  numerous 
short  branches  at  the  line  of  juncture  with  the  enamel ;  they  are  dilated, 
and  a  little  bent  out  of  their  course,  while  beyond  the  dilatation  they  pass 
on  through  about  two-thirds  of  the  thickness  of  the  enamel  in  a  straight 
course  and  without  branches.  Only  a  part  of  the  whole  thickness  of  the 
namel  is  shown  in  the  figure. 


THE   DEXTAL    TISSUES. 


Waldeyer  and.  Hertz  doubt  the  passage  of  the  tubes  of  the 
dentiue  into  the  enamel ;  as  KiJlliker  ubsen^es,  it  is  difficult 
to  see  how  they  can  doubt  it,  even  after  mere  observation  of 
a  single  specimen ;  moreover,  it  is  also  capable  of  experi- 
mental demonstration,  for  if  an  acid  capable  of  removing  the 
enamel  be  applied  to  one  of  these  sections  of  marsupial  teeth 
so  as  to  dissolve  away  the  enamel,  the  fi*eed  tubes  are  left 
hanging  out  from  the  edge  of  the  dentine,  thus  putting  the 
matter  beyond  aU  possibility  of  doubt,  while  the  develop- 
ment of  the  marsupial  enamel  makes  the  nature  of  the  con- 
tents of  the  tubes  quite  clear. 

The  most  marked  variation  in  the  structui-e  of  enamel, 
which  is  on  the  whole  a  tissue  differing  but  little  in  various 
animals,  is  met  with  in  the  class  of  fish. 

In  the  Sargus,  or  sheep's-head  fish,  for  example,  the  enamel 
is  penetrated  by  a  system  of  tubes  which  are  not  continued 


Fis.  2-1  ('). 


K 


MWMMm 


Dut  of  or  derived  from  the  dentine,  but  belong  to  the  enamel 
itself. 

The  tubes,  as  seen  in  the  figure,  run  at  right  angles  to 
the  external  surface  of  the  enamel,  px-oceed  inwards  without 
branch  or  bend  for  some  little  distance,  and  then,  at  about 

(')  Enamel  and  dentine  of  the  Sheep's-head  fish  (Sargus  oris). 
The  enamel  is  penetrated  by  a  system  of  channels  which  enter  from  its 
free  exposed  surface,  pass  in  for  a  certain  distance  in  straight  lines,  and 
then  abruptly  Lending  at  an  angle  cross  one  another,  and  produce  a  com- 
plicated pattern  in  the  inner  third  of  the  enamel. 


56  A    MANUAL    OF  DENTAL   ANATOMY. 

the  same  point,  bend  abruptly  at  an  angle,  and  give  oft 
numerous  branches.  The  meshwork  produced  by  the  cross- 
ing of  the  tubes  at  all  sorts  of  angles  in  the  inner  part  of 
the  enamel  is  so  complicated  as  to  render  it  impracticable 
to  reproduce  it  in  a  drawing.  That  portion  of  enamel  next 
to  the  dentine  is  without  canals.  Von  Boas  (Zeits,  f.  wissen. 
Zoolog.  Bd.  xxxii.),  describing  the  similarly  constructed 
enamel  of  scaroid  fishes,  says  that  I  was  in  error  in  sup- 
posing that  the  canals  open  upon  the  outer  siu'face  of  the 
enamel.  But  I  do  not  understand  his  reasons  for  dissenting 
from  my  opinion,  which  re-examination  of  many  specimens 
has  tended  to  confirm.  I  have  not  been  able  to  satisfy 
myself  whether  the  tubes  occupy  the  interspaces  of  the 
enamel  prisms,  or  their  axes. 

It  would  appear  also  as  if  these  tubes  were  empty  during 
life,  as  in  sections  they  appear  to  be  more  or  less  blocked  up 
with  dirt.  The  existence  of  the  prisms  at  all  is  not  certain, 
and  this  led  Kolliker  to  say  that  true  enamel  does  not 
appear  to  exist  in  fishes  (Mik.  Anat.  p.  114);  the  enamel  of 
fish  is,  however,  developed  from  an  enamel  organ  homologous 
with,  and  exactly  like,  that  of  amphibia  and  reptiles,  so 
that  these  anomalous  tissues  must  1)e  regarded  as  l)eing  un- 
questionably enamel. 


The  greater  part  of  every  tooth  is  made  up  of  dentine, 
which  thus,  even  after  the  removal  of  the  other  tissues 
would  preserve  somewhat  its  characteristic  form.  Several 
varieties  of  dentine  exist  in  which  those  peculiarities  of 
structure  which  differentiate  it  from  bone  become  less 
marked,  so  that  a  point  is  sometimes  reached  at  which  it 
is  hard  to  say  whether  a  particular  structure  should  more 
rightly  be  regarded  as  dentine,  or  as  bone.  It  will  be  most 
convenient  to  commence  with  the  description  of  that  variety 


THE   DENTAL    TISSUES.  57 

of  dentine  which  differs  most  markedly  from  bone  ;  in  other 
words,  which  has  the  most  typical  "  dentinal  "  structure  ; 
and  for  that  purpose  the  tissue  met  with  in  the  teeth  of  man 
and  the  majority  of  mammalia,  (though  it  is  by  no  means 
confined  to  that  class,)  and  known  under  the  name  "hard" 
or  "unvascular"  dentine  may  be  selected. 

Dentine  is  a  hard,  highly  elastic  substance,  in  colour 
white  with  a  slight  tinge  of  yellow,  and  to  some  extent 
translucent,  its  transparency  being  often  made  more  striking- 
by  contrast  with  the  opacity  which  marks  the  first  advent  of 
dental  caries,  ^yheu  broken  a  silky  lustre  is  seen  upon  the 
fractured  surfaces,  which  being  in  the  main  due  to  the  presence 
of  air  in  its  tubes,  is  more  apparent  in  dry  than  in  fresh 
dentine ;  its  fracture  is  sometimes  described  as  finely  fibrous. 

The  mass  of  the  dentine  consists  of  an  organic  matrix 
richly  impregnated  with  calcareous  salts ;  this  matrix  is 
everywhere  permeated  by  parallel  tubes,  which  run,  with 
some  deviations,  in  a  direction  at  right  angles  to  the  siu'face 
of  the  tooth. 

The  Matrix. — The  exact  chemical  composition  of  the 
matrix  is  not  known ;  in  man  the  proportion  borne  by  the 
organic  to  the  inorganic  constituents  varies  in  diffei'ent 
individuals,  and  very  probably  in  the  same  individual  at 
different  ages,  so  that  analyses  can  only  give  approximate 
results.  In  a  fresh  human  tooth  62  per  cent,  of  its  weight 
was  found  to  be  inorganic  salts,  the  tooth  cartilage  being 
28  per  cent.,  leaving  a  residue  of  10  per  cent,  of  water. 

Von  Bibra  gives  the  following  analysis  of  perfectly  dried 
dentine  : — 

Organic  matter  (tooth  cartilage)  .     27"61 

Fat 0-40 

Calcium  phosphate,  and  fluoride  .  06" 72 
Calcium  carbonate  .  .  .  .  3  "36 
Magnesium  phosphate  .  .  .  .  1"18 
Other  salts 0-83 


58 


A    MANUAL    OF  DENTAL    ANATOMY. 


Von  Bibra  gives  another  analysis  : — 

Cartilage 

20-42 

Fat 

•58 

Salts 

1-00 

Magnesium  phosphate  . 

2-49 

Calcium  phosphate,  and  fluoride 

G7-54 

Calcium  carbonate     . 

.       7-97 

And  Berzelius  gives 


Gelatine  and  water    . 
Sodium  salts 
Magnesium  phosphate 
Calcium  phosphate 
Calcium  fluoride 
Calcium  carbonate 


28-00 
1-50 
1-00 

62-00 
2-00 
5-50 


The  dentine  of  many  mammals  is  very  much  more  rich 
in  magnesium  phosphate  than  human  dentine  is;  even 
the  latter,  it  would  seem,  from  the  discrepancies  existing 
between  the  various  analyses,  is  variable  in  composition, 
but,  on  the  whole,  it  may  be  said  that,  amongst  inorganic 
constituents  of  dentine,  calcium  phosphate  largely  prepon- 
derates ;  from  3|  to  8  per  cent,  consists  of  calcium  carbo- 
nate ;  a  much  smaller  proportion  consists  of  magnesium 
phosphate,  while  calcium  fluoride  exists  in  traces  only. 

The  organic  basis  of  the  matrix  is  closely  related  to  that 
of  bone,  with  which  however  it  is  not  identical ;  it  is  of 
firmer  consistence,  and  does  not  really  yield  gelatine  on 
boiling,  but,  according  to  Kolliker  (who  quotes  Hoppe),  the 
dentine  of  the  pig  yields  a  substance  resembling  glutin, 
the  dentinal  globules  remaining  undissolved.  The  animal 
basis  of  the  dentine  is  called  "  dentine  cartilage,"  and  is 
readily  obtainable  by  submitting  a  tooth  for  several  days  to 
the   action   of  diluted  acids.     The  form  and  most  of  the 


THE   DENTAL    TISSUES.  59 

structural  characteristics  of  a  tooth  so  treated  are  main- 
tained, the  dental  cartilage  forming  a  tough,  flexible,  and 
elastic  semi-transparent  mass. 

In  the  matrix  of  a  perfect  tooth  no  trace  of  cellular 
structure  can  be  detected ;  it  is  uniform  and  perfectly 
transparent. 

The  Dentinal  Tubes. — As  has  been  already  mentioned, 
the  matrix  is  everywhere  permeated  by  tubes,  the  precise 
direction  of  which  varies  in  different  parts  of  the  tooth, 
so  that  the  following  description  of  their  com-se  must  be 
taken  as  merely  in  a  general  way  descriptive,  and  not  as 
of  universal  or  precise  application.  Each  tube  starts  by  an 
open  circular  mouth  upon  the  surface  of  the  pulp  cavity ; 
thence  it  runs  outwards,  in  a  direction  generally  per- 
pendicular to  the  surface,  towards  the  periphery  of  the 
dentine,  which,  however,  it  does  not  reach,  as  it  becomes 
smaller,  and  breaks  up  into  branches  at  a  little  distance 
beneath  the  surface  of  the  dentine. 

Near  to  the  pulp  they  are  so  closely  packed  that  there  is 
little  room  between  them  for  tlie  matrix,  while  near  to  the 
outside  of  the  tooth  they  are  more  widely  separated :  their 
diameter  is  also  greater  near  to  the  pulp  cavity. 

The  dentinal  tubes  do  not  pursue  a  perfectly  straight 
course,  but  describe  curves  both  on  a  larger  and  a  smaller 
scale.  The  longer  curves  are  less  abrupt  than  the  others, 
and  are  termed  the  "  primary  curvatures  ; "  they  are  often 
compared  to  the  letter  /,  to  which  they  bear  a  certain 
amount  of  resemblance ;  the  primary  curves  are  more 
pronounced  in  the  crown  than  in  the  root. 

The  secondary  curvatures  are  very  much  more  numerous 
and  are  smaller ;  the  actual  course  of  the  dentinal  tube  is, 
in  many  places  at  all  events,  an  elongated  spiral,  as  may 
be  very  w^ell  seen  in  thick  sections  transverse  to  the  tubes  ; 
by  alterations  in  the  focus  of  the  microscope  the  appearance 
of  the  tube  making  a   spiral  turn   is   made   veiy   striking. 


60 


A    MANUAL    OF   DENTAL    ANATOMY. 


The  effect  of  an  elongated  spiral  viewed  on  its  side  will  of 
course  be  only  slight  undulations,  such  as  are  the  secondary- 
curvatures  of  the  tubes.  The  spiral  course  of  the  dentinal 
tubes  is  most  strongly  marked  in  the  roots  of  teeth. 

When  a  transverse  section  of  dentine  is  viewed,  bands  or 
rings,  concentric  with  the  pulp  cavity  are  seen,  and  the  same 
bands  may  be  seen  in  longitudinal  section.  Such  a  striated 
or  laminated  appearance  in  the  dentine  may  be  due  to  two 
causes ;  and  some  little  confusion  has  arisen  in  the  nomen- 
clature, owing  to  its  double  origin  not  having  always  been 
kept  in  view.  Such  striae  may  be  due  to  the  presence  of 
rows  of' interglobular  spaces,  or  to  the  coincidence  of  the 
primary  curvatures  of  neighbouring   dentinal   tubes :  that 

Fig.  25  (')• 


is  to  say,  each  tube  bends  at  the  same  distance  from  the 
surface,  and  the  bend  makes  a  difference  in  the  optical  pro- 
perties of  the  dentine  at  that  point. 

Schreger  described   these  latter:  the  lines  of  Schreger, 


(')  Dentine  and  cementum  of  a  Ntirwal,  showing  contour  lines  due  to 
TOWS  of  interglobular  spaces. 


THE   DENTAL    TISSUES'. 


61 


therefore,  are  markings,  ranged  parallel  with  the  exterior  of 
the  dentine,  which  are  due  to  the  curvatures  of  the  dentinal 
tubes. 

The  "  contour  lines  "  of  Owen,  even  in  his  own  works, 
include  markings  of  both  classes  :  i.  e.,  those  due  to  the 
curvature  of  the  dentinal  tubes,  and  those  due  to  laminrc  of 
interglobular  spaces,  such  as  are  met  with  in  the  teeth  of 
Cetacea.  Retzius  had  seen  and  described  contour  markings 
due  to  interglobular  spaces,  though  his  name  is  not  usually 
associated  with  them,  the  "  brown  strise  of  Retzius  "  being- 
markings  in  the  enamel. 

The  tubes  as  they  pass  outwards  often  divide  into  two 
equally  large  branches ;  they  also  give  off  fine  branches, 
which  anastomose  with  those  of  neighbouring  tubes.     In  the 

Fig.  26  C). 


cro^vn  of  a  human  tooth  these  fine  branches  are  compara- 
tively few,  until  the  tube  has  reached  nearly  to  the  enamel, 
but  in  the  root  they  are  so  numerous  as  to  afford  a  ready 
means  of  distinguishing  whence  the  section  has  been  taken. 
The  small  branches  above  alluded  to  are  given  off  at  right 
angles  to  the  course  of  the  main  tube,  which,  however,  itself 
frequently  divides  and  subdivides,  its  divisions  pursuing  a 
nearly  para,llel  course. 

The   tubes  are   subject   to   slight    varicosities,   and   their 

(')  Temiinatioii  of  a  dentiual  tube  in  tlie  midst  of  the  dentine — human. 


62  A    MANUAL    OF   DENTAL    ANATOMY. 


coui'so  is  sometimes  apparently  interrupted  by  a  small  inter- 
globular space,  as  is  to  be  seen  in  an  extreme  degree  in  the 
dentine  of  Cetacea. 

Owing  to  their  breaking  up  into  minute  branches,  some 
of  the  tiibes  become  lost  as  they  approach  the  surface  of  the 
dentine,  and  apparently  end  in  fine  pointed  extremities. 

Some  terminate  by  anastomosing  with  terminal  branches 
of  others,  forming  loops  near  to  the  surface  of  the  dentine  ; 
others  terminate  far  beneath  the  surface  in  a  similar  way. 

Some  tubes  pass  into  the  small  interglobular  spaces 
which  constitute  the  "granular  layer"  described  by  my 
father,  while  others  again  pass  out  altogether  beyond  the 
boundary  of  the  dentine  and  anastomose  with  the  canaliculi 
of  the  lacunoQ  in  the  cementum. 

The  enamel  also  may  be  penetrated  by  the  dentinal 
tubes,  thovigh  this  Avhen  occurring  hi  the  human  subject 
must  be  regarded  as  exceptional  and  almost  pathological 
in  its  nature  (see  Fig.  22).  As  has,  however,  been  men- 
tioned in  speaking  of  the  enamel,  in  most  of  the  Marsupials 
and  in  certain  other  animals  it  is  a  perfectly  nomial  and 
indeed  characteristic  occurrence,  difficult  though  it  be  to 
see  how  such  a  relation  of  parts  is  brought  about  in  the 
course  of  development  of  the  two  tissues. 

Bentinal  Sheaths. — If  dentine  be^exposed  to  the  action 
of  strong  acid  for  some  days,  a  sort  of  fibrous  felt,  or  if  the 
action  of  the  acid  has  gone  further,  a  transparent  slime  alone 
remains.  Examined  with  the  microscope  this  proves  to  be  a 
collection  of  tubes  ;  it  is,  in  fact,  made  up  of  the  immediate 
walls  of  the  dentinal  tubes,  the  intervening  matrix  having 
been  wholly  destroyed. 

Two  facts  are  thus  demonstrated :  the  one  that  the  tubes 
Lave  definite  walls,  and  are  not  simple  channels  in  the 
matrix ;  the  other,  that  these  walls  are  composed  of  some- 
thing singularly  indestructible.  Indeed,  the  walls  of  the 
dentinal    tubes    are    so    indestructible    that    they  may  be 


THE   DENTAL    TISSUES. 


demonstrated  in  fossil  teeth,  in  teeth  boiled  in  caustic 
alkalis,  or  in  teeth  which  have  been  allowed  to  putrefy. 

Although  KolHker  was,  I  believe,  the  first  to  describe  and 
figure  these  isolated  tubes,  they  are  generally  known  as  the 
"  dentinal  sheaths  of  Neumann,"  the  latter  writer  having- 
more  fully  investigated  and  described  them.  The  precise 
chemical  nature  of  these  sheaths  will  be  more  conveniently 
considered  under  the  head  of  calcification  :  similarly  inde- 
structible tissues  are,  however,  to  be  met  with  surrounding 
the  Haversian  canals  and  the  lacunse  of  bone.  It  is  the 
opinion  of  Neumann,  as  it  was  also  of  Henle,  that  the 
dentinal  sheaths  are  calcified ;  but  the  proof  of  this  is. 
very  difficult,  as  they  cannot  be  demonstrated,  or  I  should 
rather  say,  isolated,  to  any  extent  in  dentine,  unless  it  has 
been  decalcified.  Their  existence  has  been  recently  denied 
in  toto  by  Magitot. 

Transverse  sections  of  dentine  present  fallacious  appear- 
ances, owing  to  the  thickness  of  the  section  giving  to  the 
tube  a  double  contour  which  may  be  easily  mistaken  for  a 


Fig.  '27  {^). 


special  wall.  Immediately  I'ound  the  opening  of  the  canal,. 
or  "lumen,"  as  it  is  called,  there  is  however  generally  a 
thin  yellowish  border,  which  is  the  sheath  of  Neumann. 
In  the  earlier  stages  of  caries,  before  the  dentine  is  much 
softened,  the  walls  of   the  canals  become  strikingly  appa- 

(1)  Transvei-se  section  of  dentine.     The  appearance  of  a  double  contour 
is  so  much  exaggerated  as  to  make  the  figure  almost  diagrammatic. 


64  A    MANUAL    OF   DENTAL    ANATOMY. 

rent.  The  canals  which  everywhere  permeate  the  dentine 
are  not  empty,  a  fact  which  might  be  inferred  from  the 
difference  in  translucency  and  general  aspect  of  dry  and 
fresh  dentine,  whether  seen  in  mass  or  in  thin  section  ; 
neither  are  they,  as  was  at  one  time  supposed,  tenanted 
merely  by  fluid. 

Dentinal    Fibrils. — Each  canal  is  occupied  by  a  soft 
fibril,  which  is  continuous  with  an  odontoblast  cell  upon  the 

Fir,.  28  (1). 


surface  of  the  pulp ;  the  existence  of  these  soft  fibrils  was 
first  demonstrated  by  my  father,  Avho  thus,  to  use  the  words 
of  Waldeyer,  "oj)ened  the  way  to  a  correct  interpretation 
of  the  nature  of  the  dentine." 

Henle,  in  his  "AUgemeine  Anatomic,"  (1841),  a  transla- 
tion of  a  portion  of  which  is  to  be  found  in  the  "Archives 
of  Dentistr}^,"  (1865),  figured  and  described  projections  from 
the  edges  of  fragments  of  dentine  in  continuity  with  the 
dentinal  tubes.  These  he  distinctly  describes  as  calcified  and 
rigid,  adding  that  by  the  use  of  acids  they  may  be  made 
flexible ;  he  speaks  of  the  tube  as  empty,  save  when  blocked 
by  granular  calcareous  matter,  and  alludes  to  fluids  entering 
it  by  capillarity ;  and  lastly,  he  says  nothing  whatever  of 
the  connections  of  the  pulp  Avith  the  tubes. 

Miiller,  (as  translated  in  Nasmyth  on  the  "  Structure  of 

(^)  A  fragment  of  dentine  [a),  through  which  run  the  softer  fibrils  (c), 
which  are  seen  to  be  continuous  with  tlie  odontoblast  cells  [h).  (After 
Dr.  Lionel  Beale.) 


THE   DENTAL    TISSUES.  65 

the  Teeth,"  1839),  says,  "m  breaking  fine  sections  of  the 
teeth  perpendicularly  to  the  fibres,  he  has  frequently  seen  the 
latter  projecting  a  little  at  the  fractured  edge.  In  such 
cases  they  are  quite  straight  and  not  curved,  and  seem 
to  be  not  at  all  flexible.  Hence  it  follows  that  the  tubes 
have  an  organised  basis,  a  membrane,  and  that  this  is  stiff 
and  brittle,  and  probably  saturated  with  calcareous  salts, 
but  weak  and  soft  in  a  decalcified  tooth." 

The  whole  importance  of  my  father's  discovery  la}'-  in  the 
fact  that  dentine  is  permeated  by  soft,  uncalcified  structures  ; 
and  what  is  yet  more  significant,  that  these  soft  fibrils,  per- 
meating the  hard  dentine,  proceed  from  the  pulp.  In  no 
sense,  therefore,  did  Henle  anticipate  this  discovery. 

In  1854  Lent  figui-ed  processes  from  the  dentinal  cells 
(odontoblasts)  which  he  rightly  conceived  to  be  concerned 
in  the  formation  of  dentine ;  but  in  the  earlier  editions  of 
the  "Histology"  of  his  friend  and  teacher,  Prof  Kolliker, 
although  Lent's  discoveries  are  described  and  adopted  with- 
out reservation,  no  mention  of  the  real  structure  of  dentine 
occurs.  But  in  the  last  edition,  Prof  KiJlliker  says — "  after 
Tomes  had  described  a  soft  fibre  in  each  tube,  I  fell  into  the 
mistake  of  supposing  that  these  fibres  and  the  tubes  were 
one  and  the  same." 

The  circumstances  under  which  the  dentinal  fibrils  can 
or  cannot  be  discovered  ai-e  as  follows,  and  may  be  taken  as 
proofs  of  the  distinction  between  the  dentinal  fibrils  and  the 
dentinal  sheaths. 

If  a  tooth  section  be  submitted  to  the  action  of  a  caustic 
alkali  and  boiled  in  it,  or  be  allowed  to  completely  putrefy, 
so  that  the  soft  parts  are  entirely  destroyed,  the  dentinal 
sheaths  can  still  be  demonstrated,  but  the  fibres  can  in  no 
way  be  brought  into  view  (Kolliker).  The  dentinal  sheaths 
may  be  demonstrated  also  in  fossil  teeth,  as  has  been  shown 
by  Hoppe  (Wurzburg  Nat.  Zeitschrift,  Bd.  VI.  p.  xi.)  and. 
others. 


A    MANUAL    OF   DENTAL    ANATOMY 


In  fresh  dentine  every  formative  cell  sends  a  process  into 
the  dentinal  tubes  (Tomes,  Kiilliker,  Lent,  Waldeyer,  Neu- 
mann), and  it  has  been  fovmd  possible  to  demonstrate  both 
the  sheaths  and  the  fibres  in  the  same  sections  (Neumann, 
Boll). 

In  transverse  and  even  in  longitudinal  sections  of  decalci- 
fied dentine  the  fibrils  may  be  recognised  in  situ  (Kcilliker). 

The  contrast  between  the  dentinal  sheaths  and  the  fibrils 
is  this  : — the  sheaths  are  very  indestructible,  and  can  be 
demonstrated  in  teeth  which    have  undergone  all  sorts  of 

Fig.  29  (i). 


change ;  the  soft  fibril  is  no  longer  demonstrable  when  the 
tooth  has  been  placed  in  circumstances  which  would  lead 
to  its  soft  parts  perishing.  In  dentine,  then,  we  have  (i.)  a 
matrix  permeated  by  tubes ;  (ii.)  special  walls  to  these  tubes 
or  "dentinal  sheaths;"  and  (iii.)  soft  fibrils  contained  in 
these  tubes,  or  "  dentinal  fibres ;  "  and  it  now  remains  to 
consider  these  in  farther  detail. 

In  fortunate  sections  of  small  fragments  of  dentine  taken 
from  the  edges  of  the  pulp  cavity  and  including  the  surface 
of  the  pulp,  the  dentinal  fibrils  may  be  seen  stretching  from 
the  cells  of  the  superficial  layer  of  the  pulp  (odontoblasts) 
into  the  dentinal  tubes,  as  owing  to  these  being  extensile 
they  may  be  stretched  or  drawn  out  from  the  tubes  for  some 
little  distance  without  being  broken  across.     In  the  same 

(')  Section  of  dentine  from  tlie  edge  of  which  hang  out  the  dentinal 
sheaths,  and  beyond  these  again  tlie  fibrils  (after  Boll). 


THE   DENTAL    TISSUES.  67 


way  they  may  be  seen  stretching  across  like  harp  strings 
between  two  pieces  of  dentine,  when  this  is  torn  by  needles, 
and  they  can  be  thus  shown  in  fresh  fragments  just  as  well 
as  in  those  of  decalcified  dentine.  When  stretched  to  a 
considerable  extent  their  diameter  becomes  diminished  and 
they  finally  break,  a  sort  of  bead  sometimes  appearing  at  the 
broken  end  (Tomes).  This  would  seem  to  indicate  that  the 
substance  of  the  fibril  is  of  colloid  consistency,  and  that  its 
external  portions  are  in  some  degree  firmer  than  its  axial 
portion. 

The  dentinal  fibrils  are  well  seen  in  the  accompanying 
figui'e,  in  which  some  hang  out  from  the  edge  of  the  dentine, 


PflllPf 


while  others  have  been  pulled  out  from  the  dentine  and  are 
seen  attached  to  the  odontoblast  cells. 

The  dentinal  fibril  is  capable  of  being  stained  with  car- 
mine, though  with  some  difl&culty;  in  young  dentine  it 
is  more  easily  stained,  especially  near  the  pulp  cavity,  and 
the  accompanying  drawing  is  taken  from  such  a  section  of 

(^)  Surface  of  the  pulp,  with  the  odontoblast  layer  in  situ.  The  dentine 
fibrils  pulled  out  of  the  dentinal  tube  hang  like  a  fringe  from  the  odonto- 
blast layer  :  dentine  fibrils  are  also  seen  hanging  out  from  the  edge  of  the 
dentine,  to  which,  to  the  right  of  the  figure,  a  few  odontoblasts  remain, 
attached. 

P  2 


A    MANUAL    OF  DENTAL   ANATOMY. 


dentine  from  a  half-formed  human  incisor.  The  matrix  is 
slightly  stained  with  the  carmine,  indicating  that  it  has  not 
yet  become  fidly  impregnated  with  salts,  and  in  the  centres 
of  the  clear  areas  dark  spots  deeply  stained  with  carmine 
are  to  be  seen,  the  latter  being  transverse  sections  of  the 
dentinal  fibrils  in  situ.  I  have  observed  precisely  similar 
appearances  in  the  thin  young  dentine  of  calves'  and  pigs' 
teeth ;  Kblliker  also  mentions  that  the  dentinal  fibril  may 
be  recognised  in  situ  in  transverse  sections  of  fresh  dentine. 

Fig.  31  (1). 


Bodecker  finds  that  the  dentinal  fibrils  stain  darkly  with 
chloride  of  gold ;  when  viewed  in  transverse  sections  under 
a  magnifying  power  of  2,000  diameters  they  do  not  appear 
round  but  somewhat  angular,  and  give  oflF  tiny  lateral 
offshoots  which  seem  to  penetrate  the  dentine.  In  the 
matrix  itself  there  is  an  appearance  of  a  faint  network  when 
it  has  been  stained  with  gold,  and  from  this  Bodecker  infers 
that  the  dentine  is  penetrated  everywhere  by  a  network 
of  living  plasm,  derived  from,  though  far  finer  than,  the 
dentinal  fibrils. 

Probably  the  angularity  of  the  fibril,  wdiich,  as  figured  by 
him,  is  much  smaller  than  the  canal,  is  due  to  its  having 
shrunk  under  the  action  of  chromic  acid  or  some  such 
reagent. 

(')  Transverse  section  of  dentine  :  in  four  of  the  dentinal  tubes,  the 
dentinal  fibrils  deeply  stained  with  carmine,  in  the  preparation  from  which 
this  figure  was  drawn,  are  seen.  The  fibrils  are  somewhat  shrunken, 
owing  to  the  action  of  the  glycerine  in  which  the  section  is  mounted. 


THE  DENTAL    TISSUES. 


According  to  Neumanu,  in  old  age  the  fibrils  atrophy  or 
become  calcified ;  some  observers  have  failed  to  detect  them 
near  to  the  periphery  of  the  dentine,  far  away  from  the  pulp 
cavity.  But  here  they  would  naturally  be  more  minvite, 
and  it  is  more  probable  that  the  manipulations  had  failed 
to  demonstrate  them  than  that  they  were  absent ;  for 
Bodecker  has  traced  them  to  the  very  outside  of  the 
dentine. 

Dr.  Beale  has  seen  prolongations  of  the  nucleus  of  the  cell 
toAvards  the  base  of  the  fibril,  though  in  the  example  which 
he  figures  it  does  not  enter  it. 

Dentinal  fibrils  have  been  demonstrated  in  the  Reptilia 
and  Amphibia  by  Santi  Sirena  and  myself;  and  by  myself 
in  the  few  fish  that  I  have  examined  with  that  purpose. 

Of  their  real  nature  some  doubts  are  entertained  :  they 
are  certainly  processes  of  the  formative  cells  of  the  dentine, 
and  their  substance  seems  identical  with  that  of  the  proto- 
plasm of  the  cell.  Nerves,  in  the  ordinary  sense  of  the 
word,  they  are  not,  and  have  never  been  supposed  to  be ; 
but  there  are  many  examples  of  cellular  structures  which 
are  comiected  with  the  termination  of  sensory  nerve  fibres, 
such  as  the  goblet  cells  in  the  olfactory  membrane  of  the 
frog,  and  it  is  quite  possible  that  the  odontoblast  cells  may 
stand  in  some  such  relations  to  the  nerve  of  the  pulp,  the 
termination  of  which  have  never  been  satisfactorily  traced. 

Mr.  Coleman  once  suggested  that  it  was  possible  that 
the  odontoblasts  might  have  some  tactile  function ;  but 
M.  Magitot  has  recently  claimed  for  them  a  very  definite 
connection  with  the  nerves  of  the  pulp.  According 
to  his  observations  and  figures  the  nerves  of  the  pulp 
become  continuous  with  a  layer  of  reticulate  cells  which  lie 
beneath  the  odontoblasts  ;  and  these  freely  communicate 
with  the  processes  of  the  odontoblasts,  so  that  there  is  a 
very  direct  chain  of  communication  between  the  dentinal 
fibril  and  the  nerves  of  the  pulp.     M.  Magitot  speaks  very 


70  A    MANUAL    OF  DENTAL    ANATOMY. 


positively  as  to  the  accuracy  of  his  views,  which  as  yet, 
however,  have  not  been  confirmed  by  other  investigators. 


Fig.  3-2  f»). 


U4-L^:;ii}A 


Yet  another  view  of  the  nature  of  the  dentinal  fibril  is 
advocated  by  Klein  ("Atlas  of  Histology,"  p.  183),  who 
holds  that  the  odontoblasts  are  concerned  only  in  the  forma- 
tion of  the  dentine  matrix,  and  that  the  dentinal  fibrils  are 
long  processes  of  the  deeper  cells,  in  the  above  figure,  which 
run  up  between  the  odontoblasts  and  enter  the  dental  canals. 

In  a  recent  paper  (Comptes  Rendus,  1880,)  Magitot  also 
impugns  the  accuracy  of  the  views  ordinarily  accepted  as  to 
the  structure  of  dentine,  denying  the  existence  of  any 
special  walls  to  the  tubes,  and  further  arguing  that  it  is 
undesirable  to  think  or  speak  of  the  channels  in  dried  dentine 
as  tubes  at  all.  For,  he  argues,  they  are  not  tubes  in  the 
fresh  state,  seeing  that  the  fibrils  are  adherent  to  the 
matrix  and  form  a  part  of  it,  and  that  they  were  originally 

(^)  After  Magitot.  «.  Dentinal  fibrils.  h.  Amori^lious  matrix. 
c.  OdontoljJasts.  cl.  Nuclei  of  odontoblasts,  e.  Stellate  cells.  /.  Nerve 
extremities  which  are  continuous  with  the  branched  cells. 


THE   DENTAL    TISSUES.  71 

precisely  the  same  tissue.  He  would  prefer  to  speak  of 
dentine  as  being  a  fibrillar  tissue  included  in  a  hard  and 
homogeneous  matrix. 

These  views,  however,  do  not  differ  substantially  from 
those  in  the  text,  save  that  M.  Magitot  does  not  recognise  the 
existence  of  that  transitional  tissue  which  others  believe  to 
be  there,  and  call  the  sheaths  of  Neumann. 

No  true  nerve  fibril  has  ever  been  seen  to  enter  the  den- 
tine ;  nothing  but  the  dentinal  fibril  has  ever  been  proved 
to  pass  from  the  pulp  into  the  hard  substance  of  the 
tooth  ;  nevertheless,  the  observation  of  Boll  is  very  sug- 
gestive. He  found  that  by  treating  a  perfectly  fresh  pulp 
with  ^  per  cent,  solution  of  chromic  acid  an  immense 
number  of  fine  fibres  could  be  demonstrated,  a  great  many 
of  which  projected  from  above  the  surface,  as  though  they 
had  been  pulled  out  of  the  dentinal  tubes ;  but  although 
they  pass  up  fi'om  a  plexus  of  dark-bordered  nerve  fibres 
beneath  the  membrana  eboris,  between  the  cells  of  that 
layer,  their  passage  into  the  dentine  remains  a  mere  matter 
of  inference. 

Boll's  observations  likewise  are  awaiting  confirmation  or 
disproof,  and  so  far  stand  alone. 

Be  that  as  it  may,  there  can  be  no  question  that  the 
sensitiveness  of  the  dentine  is  due  to  the  presence  of  soft 
organized  tissue  in  the  tubes,  and  is  not  a  mere  transmission 
of  vibrations  to  the  pulp  through  a  fluid  or  other  inert 
conductor.  The  peripheral  sensitiveness  of  a  tooth  can  be 
allayed  by  local  applications  which  it  would  be  absurd  to 
suppose  were  themselves  conducted  to  the  pulp ;  moreover, 
it  is  within  the  experience  of  every  operator  that  after  the 
removal  of  a  very  sensitive  layer  of  caries,  you  often  come 
down  upon  dentine,  which,  though  nearer  to  the  pulp,  is 
far  less  sensitive,  a  condition  quite  inexplicable,  except 
upon  the  supposition  of  a  different  local  condition  of  the 
contents  of  the  tubes.     Irritation  applied  to  the  dentinal 


72  A    MANUAL    OF  DENTAL    ANATOMY. 


fibrils  may  be  propagated  to  the  pulp,  and  irritation  of  the 
pulp  set  up  without  any  real  exposure  of  the  latter. 

With  reference  to  the  probabilities  of  actual  uei've  fibres 
entering  the  dentinal  tubes,  it  must  be  remembered  that,  in 
those  tissues  which  are  naturally  so  thin  as  to  present 
great  facilities  for  examination,  nerves  of  a  degree  of  fine- 
ness unknown  elsewhere  have  been  demonstrated  ;  in  other 
words,  the  easier  the  tissue  is  to  investigate,  the  finer  the 
nerves  which  have  been  seen  in  it,  while  dentine  is  among 
the  most  difiicult  substances  conceivable  for  the  demonstra- 
tion of  fine  nerve  fibrils,  if  svich  exist  in  it. 

Interglobular  Spaces. — In  the  layer  of  dentine  which 
underlies  the  cement  an  immense  number  of  these  spaces 
exist,  giving  to  the  tissue  as  seen  under  a  low  power  an 
appearance  of  granularity.     On  this  account  my  father  gave 


to  this  the  name  of  the  "  granular  layer "  of  dentine  ;  on 
account  of  the  far  'greater  abundance  of  the  spaces  in  that 
situation  it  is  far  more  marked  beneath  the  cement  than 
beneath  the  enamel,  and  many  of  the  dentinal  tubes  end  in 
these  spaces. 

Although  the  name  "  interglobular  spaces  "  is  strictly  ap- 
plicable to  the  structm-es  constituting  the  granular  layer 
of  dentine,  it  was  not  to  these  that  it  was  first  applied. 
When  a  dried  section  of  dentine  is  examined,  dark  irregular 

(^)  Dentinal  tubes  terminating  in  the  spaces  of  the  granular  layer. 


THE   DENTAL    TISSUES. 


73 


spaces,  clustered  together  and  usually  most  abundant  at  a 
little  distance  below  the  surface,  ai-e  often  to  be  seen,  parti- 
cularly if  the  section  has  been  made  from  a  brownish,  im- 
perfectly developed  tooth. 

These  spaces  have  a  ragged  outline,  furnished  with  short 
pointed  processes,  and  in  favoxn-ably-prepared  sections  it 
may  be  seen  that  their  outlines  are  formed  by  portions  of 
the  surfaces  of  closely  opposed  spheres,  and  globular  con- 
tom'S  may  often  be  detected  iii  the  solid  dentine  near  to 
them,  as  is  seen  in  the  accompanying  figure,  taken  from  a 
section  boiled  in  wax  in  order  to  render  it  very  transparent. 

Although  these  large  spaces  are  very  common,  they  are 
perhaps  not  to  be  regarded  as  perfectly  normal,   but  are 


^ 


rather  indications  of  an  arrested  development  at  that  spot. 
The  occurrence  of  globular  forms  during  the  early  stages 
of  calcificfition,  will  again  be  alluded  to  in  connection  with 
the  development  of  teeth  ;   but  although  the  term  "  inter- 

(';  Interglobular  spaces  in  dentine. 


74  A    MANUAL    OF   DENTAL    ANATOMY. 


globular  "  is  thus  strictly  applicable,  the  use  of  the  word 
"  spaces  "  is  not  so  correct.  lu  dry  dentine  it  is  true  that 
they  are,  as  Czerinak  described  them,  spaces  filled  with  air  ; 
but  that  they  are  so  is  only  due  to  the  fact  that  their 
contents  are  soft,  and  shrivel  up  in  drying.  In  the  fresh 
condition  the  interglobular  "  space "  is  perfectly  full,  its 
contents  often  having  the  structural  arrangement  of  the 
rest  of  the  matrix,  or  else  consisting  of  soft  plasm ;  in 
the  former  case,  the  dentinal  tubes  pass  across  and  through 
it  without  any  interruption  or  alteration  in  their  course. 
This  fact,  as  well  as  the  soft  nature  of  tlie  contents  as  com- 
pared with  the  rest  of  the  dentine,  is  well  illustrated  by  a 
section  in  my  possession  which  was  taken  from  a  carious 

Fia.  35  (1). 


tooth,  near  to  the  alfected  surface.  In  this  the  fungus, 
leptothrix,  had  effected  an  entrance  into  some  of  the  tubes, 
giving  to  them  a  varicose  beaded  appearance,  and  causing 
their  enlargement.  But  when  it  reached  the  interglobular 
space,  the  less  amount  of  resistance,  or  possibly  the  more 
favourable  pabulum  accessible,  led  to  its  more  rapid  deve- 
lopment, so  that  the  tubes  within  the  confines  of  the  space 
lire  many  times  more  enlarged  than  those  outside ;  never- 
theless the  continuity  of  the  tubes  across  the  space  is  well 

C)  Section  of  carious  dentine,  in  which  some  of  the  tubes  are  beaded 
by  the  ingress  of  the  leptothrix,  which  has  develoioed  with  greater  freedom 
in  one  or  two  of  the  tubes  where  they  cross  the  interglobular  spaces. 


THE   DEXTAL    TISSUES.  75 

demonstrated  by  the  gro^vth  of  leptothrix  having  followed 
them  -with  exactitude. 

It  sometimes  happens  that  indications  of  spherical  forms 
and  faintly  discernible  contours  resembling  those  of  the 
interglobular  spaces  may  be  seen  in  chied  sections,  in  which 
no  actual  spaces  occur.  The  appearances  are  perhaps  pro- 
duced by  the  formation  of  an  interglobular  space,  the  con- 
tents of  which  have  subsequently  become  more  or  less 
perfectly  calcified ;  and  the  appearance  described  as  "  areolar 
dentine"  is  probably  to  be  explained  in  this  manner. 

The  exact  nature  of  the  contents  of  the  interglobular 
spaces  is  not  very  certain  :  they  may,  with  some  difficulty, 
be  tinted  by  cai-mine,  and  it  is  said  that  they  may,  like 
the  dentinal  sheaths,  be  isolated  by  the  destruction  of  the 
rest  of  the  matrix  in  acids ;  that  this  may  be  done  I  do 
not  doubt,  although  I  have  never  succeeded  in  so  isolating 
them  myself. 

Bodecker  finds  that  there  is  soft  living  plasm  abundantly 
distributed  in  the  smaller  interglobular  spaces  which  con- 
stitute the  granular  layer,  and  that  this  is  in  very  free  com- 
munication with  the  soft  fibrils  in  the  tubes  on  the  one  side, 
and  with  the  soft  contents  of  the  lacunee  and  canaliculi  of 
the  cementum  on  the  other. 

In  the  dentine  so  far  described,  which  is  that  variety  known 
as  hard  or  uuvascular  dentine,  some  degree  of  nutrition  is 
perhaps  provided  for  by  the  penetration  of  the  whole  thick- 
ness of  the  tissue  by  protoplasmic  fibres,  the  dentinal  fibrils, 
but  this  nutrition  may  be  eff'ected  in  a  different  way,  and 
there  are  other  varieties  of  dentine  known  in  which  dentinal 
fibrils  have  never  been  shown  to   exist.      For  descriptive 
purposes  I  would  classify  dentines  as 
(i.)  Hard  or  uuvascular  dentine. 
(ii.)  Plici-dentine. 
(iii.)  Vaso-dentine. 
(iv.)  Osteo-dentine. 


76 


A    MANUAL    OF   DENTAL   ANATOMY. 


Ordinary  hard  dentine  has  been  sufficiently  described  ;  of 
it  plici-dentine  is  a  variety  not  very  distinct  in  its  essential 
nature,  though  at  first  sight  widely  dissimilar. 

Flici-deutine. — In  ordinary  dentine  the  dentinal  tubes 
radiate  out  from  a  pulp  and  pulp  chamber  of  simple  form  ; 
render  complex  that  form  by  foldings  of  its  walls,  the  den- 
tinal tubes  still  running  off  at  right  angles  to  that  portion 
of  pulp  to  which  they  immediately  belong,  and  we  have 
a  "  plici-dentine."     It  is  merely  an  ordinary  dentine  and  its 

Fio.  36  (1). 


pulp  folded  up  and  wrinkled  into  a  greater  or  less  degi'ee 
of  complexity. 

In  the  teeth  of  the  Varanus  (monitor  lizard)  the  process  of 
calcification  of  the  pulp  takes  place  in  such  manner  that  in 
the  upper  half  of  the  tooth  a  cap  of  ordinary  unvascular 
dentine,  in  which  the  tubes  radiate  from  a  single  central 


i})  Section    of    Plici-ilentine    with   the    pulj)    in    situ   (Lepidosteus). 
Odontoblasts,      p.  Connective  tissue  framework  of  jjulp.     d.  Dentine. 


THE   DENTAL    TISSUES.  77 


pulp  cavity,  is  formed.  But  in  the  lower  part  of  the  tooth 
slight  longitudinal  furrows  appear  on  the  surface,  which,  on 
transverse  section,  are  seen  to  correspond  to  dippings  in  of 
the  dentine ;  and  the  dentine  is,  as  it  were,  in  folds.     The 

Fig.  .37  {'). 


pulp  on  section  might  be  compared  to  a  paddle-wheel,  the 
floats  of  which  correspond  to  the  thin  flat  radiating  pro- 
cesses of  pulp ;  but  as  yet  the  central  pulp  chamber  is 
unaltered.  A  little  lower  down,  as  represented  in  the 
accompanying  figure,  there  is  no  longer  a  central  simple 
pulp  chamber;  the  inflections  round  the  periphery  have 
become  relatively  much  deeper,  and  the  centre  of  the  tooth 
is  occupied  by  a  tissue  irregular,  but  not  otherwise  unlike 
the  dentine  of  Myliobates ;  that  is  to  say,  there  are  a 
number  of  columns  of  pulp,  each  of  which  forms  the  axis 
whence  a  system  of  dental  tubes  radiate. 

The  outrunning  plates  of  dental  pulp,  which  on  section 
radiate  out  like  the  spokes  of  a  wheel,  do  not  always  remain 
single  ;  they  may  divide  simply  into  two  branches,  as  may 
be   seen   in   the  section   across  the  base  of  the   tooth   of 


(>)  Transverse  section  across  the  cro^^•n  of  tlie  tootli  of  Yaranus,  near  to 
its  base.  The  central  pulp  cavity  is  produced  out  into  processes,  and  it 
might  be  said  the  dentine  is  arranged  in  plates  with  some  little  regularity 
round  its  periphery. 


78 


A    MANUAL    OF   DENTAL    ANATOMY 


of  Lepidosteus  (j^orth  American  bony  pike)  ;  or  sometimes 
there  are  several  branches. 


Fig.  38  (>). 


In  Lepidosteus  oxyurus  there  are  simple  inflections,  and 
a  central  pulp  cavity;  in  L.  spatula  the  inflections  are 
branched,  and  the  central  pulp  cavity  all  filled  vip. 

In  the  foregoing  figure  of  the  base  of  a  tooth  of  Lepi- 
dosteus some  few    of  the    outrunning   pulp   chambei's  are 


(^)  Transverse  section  across  the  tooth  of  Lepidosteus  spatula.  At  the 
exterior  are  regularly  disposed  radiating  plates  of  dentine,  each  with  its 
own  pulp  cavity,  while  the  central  area  is  composed  of  more  or  less  cylin- 
drical pulp  chambers,  each  of  which  forms  the  stai-ting  point  for  its  own 
system  of  dentinal  tubes.  The  pulp  chambers  are  made  dark  in  the  figure 
for  the  sake  of  greater  distinctness. 


THE   DENTAL    TISSUES. 


79 


seen  to  be  bifurcated,  Avhile  the  central  mass  of  the  tooth 
is  composed  of  dentine  permeated  by  pulp  canals  which 
pursue  a  longitudinal  coiu'se ;  a  slight  further  modification 
brings  us  to  the  structure  of  the  dentine  of  the  Labyrin- 
thodon,  in  which  a  maximum  of  complexity  is  attained, 
although  a  clue  to  its  intimate  structure  is  afforded  by  the 
teeth  of  Varanus  or  of  Lepidosteus. 

The  laminae  of  pulp,  w^ith  their  several  systems  of  den- 


Fig.  39  (i). 


tinal  tubes,  instead  of  passing  out  in  straight  lines  like  the 
spokes  of  a  wheel,  pursue  a  tortuous  course  as  they  run 
from  the  central  small  pulp  chamber  towards  the  surfiice. 
Not  only  do  they  undulate,  but  they  also  give  off  lateral 


(^)  Transverse  section  of  a  tootli  of  Labyrinthodon.     (After  Owen.) 
fl  -^The  letter  a>  is  placed  in  the  centre  jjulp  chamber  ;  the  letter  b  marks 
the  lines  of  separation  between  the  system  of  dentinal  tubes  which  belong 
to  each  lamina  of  pulp  ;  these  lines  of  demarcation  were  formerly  sup- 
posed to  be  occupied  by  cemeutum. 


80  A    MANUAL    OF   DENTAL    ANATOMY. 

processes  ;  and  at  their  terminations  near  to  the  surface  of 
the  tooth,  the  thin  laminae  of  pulp  (so  thin  that  the  radi- 
ating pulp  chambers  are  mere  fissures)  become  dilated ;  so 
that  on  section  circular  canals  are  seen  at  these  points,  as 
is  also  the  case  at  the  points  where  subsidiary  processes 
branch  off. 

The  wavy  course  pursued  by  the  radiating  plates  of  den- 


"//^  -"-    '  "^^^smf'i    ~  ■^^- 


A\ 


■t 


iiitffi£f~z=z^  --■>///|!' 


tine,  and  the  disposition  of  the  tubes  round  the  dilated 
portions  of  pulp  chamber,  render  the  general  aspect  of  the 
dentine  structure  very  complicated ;  the  several  "systems"  (-) 

(^)  From  tooth  of  Lalija-iiitliodon,  showing  the  nature  of  the  connection 
between  the  contiguous  dentinal  systems.    (After  a  di-awing  of  my  father's. ) 

(2)  The  term  "  dentinal  system  "  is  applied  to  the  portion  of  dentine  in 
which  all  the  tubes  radiate  from  a  single  section  of  pulp  chamber  ;  thus 
the  tooth  of  Labyrinthodon  is  made  up  of  many  dentinal  system.?  ;  the 
same  thing  may  be  said  of  the  tooth  of  Myliobates. 


THE   DENTAL    TISSUES. 


are  united  to  one  another  by  an  inosculation  of  the  ter- 
minal branches  of  the  tubes  in  some  few  places,  but  more 
generally  by  a  clear  layer  containing  radiate  spaces,  some- 
thing like  the  lacunae  of  cementum.  Hence  Professor  Owen 
has  described  the  tooth  as  consisting  of  radiating  plates  of 
dentine,  between  which  pass  in  equally  convoluted  plates 
of  cementum.  But,  as  was  pointed  out  by  my  father  (Phil. 
Trans.  1850),  the  mere  presence  of  lacuna-like  spaces  is 
not  sufficient  to  prove  the  presence  of  cementum,  inasmuch 
as  they  occur  on  a  small  scale  in  the  granular  layer  of 
dentine ;  moreover,  when  cementum  and  enamel  are  both 
present,  the  cementum  is  always  outside  the  enamel,  whereas 
at  the  upper  part  of  the  tooth  of  the  Labyrinthodon  the  cha- 
racteristic inflections  take  place  within  a  common  investment 
of  enamel  w^hich  does  not  dip  in.  Thus  the  whole  of  the 
tissue  constituting  the  very  complex  pattern  of  the  Laby- 
rinthodon tooth  is  dentine,  and  the  cementum  does  not,  as 
was  usually  supposed,  enter  into  its  composition  at  all. 

Another  form  in  which  plici-dentine  may  exist  is  exempli- 
fied in  the  teeth  of  Myliobates,  a  lai*ge  Ray  ;  or  in  the  teeth 
of  the  rostrum  of  the  saw-fish  (Pristis). 

In  the  Myliobates  (Fig.  41)  the  flat  pavement-like 
tooth  is  permeated  by  a  series  of  equidistant  parallel 
straight  canals,  running  up  at  right  angles  to  the  surface ; 
from  the  upper  end  and  sides  of  these  channels  systems  of 
dentinal  tubes  radiate,  just  as  the  tubes  radiate  from  the 
single  pulp  chamber  of  a  human  tooth,  save  that  they  rim 
for  a  comparatively  short  distance.  In  transverse  sections 
the  tubes  are  seen  radiating  from  these  channels,  and  at 
their  terminations  sometimes  inosculating  with  the  terminal 
branches  of  the  tubes  of  another  system.  The  channels 
contain  prolongations  of  the  vascular  pulp,  which  are 
distinct  in  the  upper  pai't  of  the  tooth,  but  intimately 
united  together  at  its  base,  where  the  disposition  of  the 
channels  ceases  to  be  regular,  and,  as  a  consequence,  the 

G 


82  A   MANUAL    OF  DENTAL    ANATOMY. 


systems  of  dentinal  tubes  pass  from  them  in  various  direc- 
tions without  producing  the  symmetrical  patterns  which 
characterise  the  upper  part  of  the  crown. 


Fig.  41 


When  the  tooth  comes  into  use  and  its  immediate  surface 
gets  worn  off,  the  ends  of  the  perpendicular  pulp  channels 
would  be  laid  open,  were  it  not  that  they  become  blocked 
by  the  deposition  of  a  transparent  homogeneous  tissue 
Avithin  them,  analogous  to  the  similar  tissue  which  closes 
Haversian  canals  of  an  antler  about  to  be  shed. 

Such  is  an  example  of  plici-dentine  in  a  simple  form,  in 
which  the  tooth  might  be  said  to  be  built  up  of  a  series  of 
small  parallel  denticles ;  and  a  similar  structure  is  pre- 
sented by  the  rostral  teeth  of  the  saw-fish,  and  by  the  teeth 
of  the  Orycteropus  or  Cape  ant-eater. 

Vaso-dentine. — In  the  dentine  of  human  teeth  it  occa- 
sionally happens  that  a  larger  canal  is  found,  having  no 
clear  relation  to  the  course  of  the  dentinal  tubes,  which  it 
crosses  at  various  angles ;  this  larger  canal  contained  the 
(^)  Transverse  section  of  the  dentine  of  Myliobates. 


THE   DENTAL    TISSUES. 


blood-vessel,  the  remains  of  which  may  be  found  even  in 
a  dried  section.  But  in  human  dentine  vascular  canals  do 
not  often  occur,  and  when  they  do,  are  to  be  regarded  as 
decided  abnormalities. 

The  accompanying  figure,  representing  a  canal  of  large 

Fig.  42  ('). 


size,  was  drawn  from  a  specimen  shown  to  me  at  the  Cam- 
bridge (Massachusetts)  Museum  Ijy  Dr.  Andrews. 

In  some  mammalian  teeth  these  vascular  canals  are 
disposed  with  regularity,  running  out  in  loops  from  the 
pulp  cavity,  and  lying,  for  a  considerable  i:)art  of  their  course, 
at  right  angles  to  the  dentinal  tubes. 

lu  the  Manatee  for  example  the  dentinal  tubes  radiate 
out  with  perfect  regularity  from  the  central  pulp  chamber, 
and,  so  to  speak,  take  no  notice  of  the  vascular   canals, 

(,')  Vascular  canal  in  dentine.     From  a  liuman  tooth. 

G  2 


84 


A    MANUAL    OF   DENTAL    ANATOMY. 


which  are  to  be  met  with  (especially  in  the  root)  in  large 
numbers. 

Where  they  are  numex'ous  the  vascular  canals  form  loops, 
so  as  to  constitute  a  sort  of  plexus  beneath  the  cementum. 

The  Tapir,  whose  teeth  in  external  configuration  are  not 
very  dissimilar  to  those  of  the  Manatee,  also  has  vascular 
canals  in  the  dentine  ;  a  curious  diftereuce  in  this  respect 

Fia.  43  (•). 


was  pointed  out  by  my  father  (Proc.  Zoolog.  Soc.  1851) 
between  the  Indian  and  the  American  Tapir,  the  former 
having  the  canals  in  the  dentine  of  the  crown  of  the  teeth, 
the  latter  having  them  not.  The  great  extinct  ^Megatherium 
possessed  dentine  very  rich  in  these  canals  :  to  the  left  of 
the  figure  is  seen  the  inner  portion  of  the  dentine,  rich  in 
them ;  in  the  middle  a  fine  tubed  dentine,  forming  the 
external  layer  of  the  dentine  of  the  whole  tooth,  and  to  the 
right  cementum,  also  rich  in  vascular  tubes. 

In  those  teeth  in  which  the  whole  pulp  is  converted  into 
solid  material,  and  no  pulp  cavity  remains,  the  last  portions 
of  the  pulp  are  often  converted  into  dentine  which  has  not 
the  same  character  as  that  of  the  rest  of  the  tooth.     Thus 


{})  Dentine  and  cementum  of  Megatherium  :  the  latter  to  the  right  of 
the  figure. 


THE   DENTAL    TISSUES. 


in  teeth  of  perpetual  growth,  such  as  the  incisors  of  rodents, 
the  axial  portion  of  the  tooth  is  that  latest  calcified,  and 
consists  of  a  dentine  containing  vascular  canals,  which  are 
not  present  in  the  other  part  of  the  tooth.  When  a  change 
thus  occm-s  in  the  character  of  the  tissue  formed  at  a  later 
time  than  the  rest  of  the  dentine,  the  name  "secondary 
dentine  "  is  applied  to  the  resultant  tissue. 

But  secondary  dentine  may  partake  of  several  different 
varieties  of  structure,  so  that  the  term  must  not  be  taken  as 

Fig.  44  (')■ 


denoting  anything  more  than  the  circumstances  under  which 
it  was  formed. 

It  is  in  the  class  of  Fish,  in  which  vaso-dentine  is  rather 
common,  that  the  most  instructive  examples  of  its  nature 
are  to  be  found. 

The  conical  teeth  of  the  common  Flounder,  and  indeed  of 
most  flat  fish  (Pleuronecticke)  have  their  pointed  tips  formed 


(*)  Tooth  of  a  Flounder,     a,  Dentinal  tubes  near  apex  of  tooth  ;  b. 
Vascular  canals  ;  c,  Spear  points  of  enamel. 


86  A    MANUAL    OF  DENTAL    ANATOMY. 


of  ordinary  hard  dentine,  surmounted  by  enamel  tips.  In 
this  part  of  the  tooth  the  dentinal  tubes  are  numerous,  and 
regular  in  their  disposition,  radiating  out  from  the  axial  i)ulp 
chamber. 

Lower  down  in  the  teeth  the  dentinal  tubes  become  less 
numerous,  and  at  the  same  time  much  lai'ger  looped  canals 
make  their  appearance,  and  as  these  become  more  numerous 
and  regular  so  do  the  dentinal  tubes  become  less  so.  These 
larger  tubes  contain  blood-vessels,  and  red  blood  circulates 
through  them  during  the  life  of  the  tooth. 

We  may  suppose  that  tha  nutrition  of  the  dentine  maybe 

Fig.  45  Q). 


provided  for  either  by  protoplasm  carried  for  a  long  distance 
from  the  pulp  by  the  dentinal  tubes,  or  by  blood  circulating 
through  the  larger  vascular  channels,  but  that  both  are  not 
required,  and  so  do  not  exist  together. 

And  whilst  the  teeth  of  the  Manatee,  the  Tapir,  and  of 

(^)  Tooth  of  Ostracion.     a,  Enamel ;  h.  Capillary  channels  ;    c,   Axial 
pulp  chamber. 


THE   DENTAL    TISSUES.  87 

the  Flounder  teach  that  hard  dentine  and  vaso-dentine  are 
not  very  dissimihir  in  their  nature,  and  tliat  the  one  passes  by 
imperceptible  gradations  into  the  other,  the  dentine  at  the 
base  of  the  Flounder's  tooth  provides  us  with  an  example  of 
typical  vaso-dentine  :  that  is  to  say,  dentine  in  which  the 
dentinal  tubes  are  quite  absent,  having  had  their  place 
taken  by  a  complete  system  of  vascular  channels. 

The  teeth  of  the  Ostracion  (Fig.    45),  or  of   the  Hake 
(Figs.  46  and  ^^),  afford  good  examples  of  this  form  of  tissue. 

Fig.  46  ('), 


'^;~\ 

■{ 

o/t-^^.---:; 

■^   ' 

r^^^f.^  __. 

C]i 


The  matrix  is  solid,  so  far  as  penetration  by  fine  tubes 
goes,  but  it  contains  a  system  of  larger  canals  which  carry 
only  blood,  and  no  pulp  tissue,  out  to  near  the  surface  of 
the  dentine,  where  they  form  a  plexus. 

I  have  not  been  able  to  satisfy  myself  of  the  existence  of 
any  definite  structure  in  the  matrix ;  sometimes  it  looks 
granular,  and  sometimes  has  a  finely  reticulated  look,  re- 
calling the  appearances  described  by  Bodecker  in  human 
dentine.     (See  page  68.) 

Q)  Section  of  Dentine  from  a  freshly  caught  Hake  (Merlucius).  d. 
Dentine  matrix  ;  cp,  Capillary  blood-vessels  hanging  out  from  its  edge, 
containing  here  and  there  abundant  blood-corpuscles. 


A    MANUAL    OF   DENTAL   ANATOMY. 


The  an-angement  of  the  vascular  canals  is  regular  and 
striking,  reminding  one  of  the  appeai'ance  of  the  capillary 
network  in  an  injected  intestinal  villus.  In  fact,  an  intes- 
tinal villus  petrified,  whilst  the  capillary  network  remained 
pervious  and  carried  red  blood  circulating  thi-ough  it,  would 
form  no  bad  representation  of  a  conical  vaso-dentine  tooth. 

For  these  canals  do  actually  contain  capillaries,  and  blood 
actively  circulates  through  them ;  a  section  cut  from  the 
fresh,  brilliantly  red  tooth  of  a  Hake  often  shows  the  coats 
of  the  capillary  hanging  out  from  the  edge,  and  the  canals 
full  of  blood-corpuscles  {Fig.  46). 

In  all  vaso-dentine  teeth  with  which  I  am  acquainted  the 
pulp  chamber  is  of  simple  form,  the  pulp  coated  by  a  distinct 
layer  of  odontoblasts,  and  no  pulp-tissue  other  than  the 
capillaries  passing  out  into  the  dentine,  so  that  each  capil- 
lary fits  and  wholly  fills  its  channel  in  the  dentine. 

Vaso-dentine  is  less  dense  and  hard  than  ordinary  dentine, 
and  consequently  generally  gets  protection  by  a  harder 
tissue  when  exposed  to  hard  work. 

The  teeth  of  the  Hake,  used  simply  for  piercing  and 
catching  fish,  are  merely  tipped  with  enamel  (Fig.  86) ; 
those  of  Ostracion,  put  to  severer  work,  are  coated  with 
enamel,  while  the  teeth  of  the  Wrasse  (Labrus),  which  are 
composed  of  ordinary  dentine  are,  though  very  hard  worked, 
unprotected  by  enamel. 

Osteo-dentine. — This  is  a  tissue  far  more  sharply  marked 
off  from  hard  dentine,  plici-dentine  and  vaso-dentine,  than 
these  are  from  one  another,  and  approaches  closely  to  bone, 
from  which  it  has  few  points  of  essential  diff"erence. 

The  distinction  can  hardly  be  fully  emphasized  until  the 
development  of  dentine  has  been  described,  but  it  may  be 
mentioned  that  it  is  not  developed  on  the  surftice  of  the  pulp, 
from  an  odontoblast  layer,  but  within  its  whole  substance. 
Consequently  in  a  completed  osteo-dentine  tooth  there  is  no 
single   simple   pulp,    which   can   be   withdrawn    from   the 


THE   DENTAL    TISSUES. 


89 


tooth,  but  pulp  and  calcified  tissue  are  quite  inextricably 
mixed  up. 

And  though   there  are    numerous   large  channels,   often 
much  larger  than  those  of  vaso-dentine,  they  are  less  regular, 


Fi;;.  47  ('). 


do  not  in  their  arrangement  suggest  the  idea  of  capillary 
loops,  and  in  a  fresh  tooth  contain  masses  of  pulp-structure 
as  Avell  as  blood-vessels. 

The  Pike's  tooth  aftbrds  a  good  example  of  osteo-dentine. 
Its  surface  is  formed  of  a  layer  of  fine  tubed  tissue,  almost 


(1)  Tooth  of  Common  Pike. 
inner  mass  of  osteo-dentiue. 


a,  Outer  la3'er  of  fine  tubed  dentine  ;  b, 


A    MANUAL    OF   DENTAL    ANATOMY. 


like  ordinary  dentine,  but  this  soon  gives  place  to  a  coarsely 
channeled  tissue,  containing  elongated  spaces  filled  with  pulp, 
from  which  canaliculi,  like  those  of  a  Ijone  lacuna,  branch 
off  in  all  directions,  but  do  not  run  far. 

Veiy  many  sharks  have  teeth  composed  of  osteo-dentine, 

Fig.  4S  (i). 


witli  an  outer  dense  layer :  the  tooth  of  Lamna  here  figured 
shows  a  central  core  of  osteo-dentine,  which  constitutes  the 
bulk  of  the  tooth ;  external  to  this  a  somewhat  thin  layer  of 
hard  dentine,  in  which  all  the  dentinal  tubes  run  out  at 
right  angles  to  the  surface,  but  are  derived  from  the  channels 
of  the  osteo-dentine  and  not  from  any  single  pnlp  chamber ; 
Avhile  the  outermost  layer,  which  is  clear  and  structureless, 

{')  Tooth  of  a  species  of  Lamna,  consisting  of  a  central  mass  of  vaso- 
dentine,  passing  towards  its  surface  into  a  fine-tubed  unvascular  dentine. 
The  clear  structureless  layer  on  the  surface  may  probably  be  regarded  as 
enamel. 


THE   DENTAL    TISSUES.  91 

may  be  merely  the  outer  part  of  the  hard  dentine,  or  may 
be  a  thin  layer  of  enamel.  It  is  to  be  regretted  that  special 
names  have  been  given  to  this  layer  ;  it  is  sometimes  called 
vitro-dentine,  sometimes  gauoin  or  fish-enamel ;  but  there  is 
no  reason  why  it  should  have  a  special  name  at  all.  The 
similarity  of  the  channels  of  pulp  in  osteo-dentine  to  Haver- 
sian canals  in  bone  is  very  close ;  in  fact,  -when  teeth  con- 
sisting of  osteo-dentine  become,  as  in  many  fish  they  do, 
anchylosed  to  the  subjacent  bone,  it  becomes  impossible  to 
say  at  what  point  the  dentine  ends  and  the  bone  commences ; 
and  this  difficidty  is  intensified  by  the  fact  that  the  bone  of 
many  fishes  lacks  lacunse,  and  is  almost  exactly  like  dentine. 

Osteo-dentine  was  defined  by  Professor  Owen  as  dentine 
in  which  the  matrix  was  arranged  in  concentric  rings  around 
the  vascular  canals,  and  in  which  lacunse  similar  to  those  of 
bone  were  found. 

But  neither  of  these  characters  are  to  be  found  in  many 
teeth,  which,  if  the  manner  of  their  development  is  to  be 
taken  into  account,  are  unquestionably  made  of  osteo-den- 
tine ;  and  so  they  cannot  be  made  use  of  for  purposes  of 
definition,  although  lacunse  and  lamination  of  the  matrix 
axe  far  moi*e  often  present  in  osteo-dentine  than  in  the  other 
varieties  of  dentinal  structure. 

The  varieties  of  dentine  may  be  grouped  thus  : — 

(A.)  Dentines  developed  upon  the  surface  of  a  pulp,  by 
calcification  of  a  specialised  layer  of  odontoblast 
cells. 

(i.)  Hard,  unvascular  dentine,  thoroughly  per- 
meated with  dentinal  tubes,  which  radiate 
from  a  simple  central  pulp  chamber. 
Example — Human  dentine, 
(ii.)  Plici-dentine,  permeated  with  dentinal 
tubes,  which  radiate  from  a  pulp  chamber 
rendered  complex  in  form  by  foldings  in 


92  A    MANUAL    OF  DENTAL    ANATOMY. 

of  its  walls.      Example — Lepidosteus, 
Labyrinthodon. 
(iii.)  Vaso-dentine,     dentinal    tubes    few    or 
absent,    but    capillary    channels    with 
blood   circulating  through  them  abim- 
dant.     Example — Hake. 
(B.)  Dentines    developed    by    calcifications     shooting- 
through  the  interior  of  a  pulp,  not  by  calcifi- 
cation of  a  specialised  surface  layer  of  cells, 
(iv.)  Osteo-dentine ;    with  no    true   dentinal 
tubes,  but  minvxte  tvibes  analogous  to 
bone    canaliculi,    and    large    irregular 
channels     containing    pulp-tissue    (not 
blood-vessels   only).      Example — Tooth 
of  Pike. 

It  remains  to  be  added  that  the  same  pulp  may  undergo 
a  change  in  the  manner  of  its  calcification ;  that  is  to  say, 
that  after  having  gone  on  with  surface  calcification  from 
an  odontoblast  layer  for  a  certain  length  of  time,  this  may 
give  place  to  a  more  irregular  internal  calcification  into  an 
osteo-dentine. 

This  is  especially  prone  to  happen  after  injury,  and  is 
often  exemplified  upon  a  large  scale  in  Elephants'  tusks  ; 
the  pulp  of  which,  normally  engaged  in  calcifying  the 
odontoblast  layers  into  ivory,  may  after  an  injury  calcify 
irregularly,  and  solidify  into  a  coarse  osteo-dentine. 

It  will  then  be  easy  to  understand  that  so-called  secon- 
dary dentine,  produced  in  a  pulp  which  ordinarily  forms 
hard  dentine,  may  partake  of  the  character  of  vaso-  or  of 
osteo-dentine. 

Thus  the  pulp  of  a  sperm  whale's  tooth  becomes  oblite- 
rated by  a  development  of  secondary  dentine,  which  some- 
times forms  irregular  masses  loose  in  the  pulp  chamber,  and 
sometimes  is  adherent  to  and  continuous  with  the  dentine 


THE   DENTAL    TISSUES. 


previously  formed.  The  structure  of  these  masses  is  very 
confused.  Tubes,  of  about  the  same  diameter  as  dentinal 
tubes,  abound  ;  but  thev  are  often  arranged  in  tufts  or  in 


bundles,  and  without  any  apparent  reference  to  any  com- 
mon points  of  radiation.  Irregular  spaces,  partaking  of  the 
character  of  interglobiilar  spaces  or  of  bone  lacunte,  abound; 
and  vascular  canals  are  also  common. 

In  the  human  tooth  secondary  dentine  occurs  in  the  teeth 
of  aged  persons,  in  which  the  pulp  cavity  is  much  contracted 
in  size,  and  is  also  very  frequently  formed  as  a  i^rotection  to 
the  pulp  when  threatened  by  the  approach  of  dental  caries, 
or  by  the  thinning  of  the  walls  of  the  pulp  cavity  through 
excessive  wear.  The  accompanying  figure,  representing  one 
of  the  cornua  of  the  pulp  chamber  from  a  molar  tooth 
affected  by  caries,  is  a  good  example  of  secondary  dentine. 

(')  Section  of  a  mass  of  secondary  dentine  from  the  tooth  of  a  sperm 
whale. 


94 


A    MANUAL    OF   DENTAL    ANATOMY. 


It  occasionally  happens  that  the  pulp  resumes  its  formative 
activity,  and  new  dentine  is  developed  which,  with  the 
exception  of  a  slight  break  or  bend  in  the  continuity  of  the 
tubes,  is  almost  exactly  like  normal  dentine.     Moi'e  often. 

Fig.  50  {}). 


however,  the  boundary  line  between  the  old  and  the  new  is 
marked  by  an  abundance  of  irregular  spaces  and  globular 
contours,  whilst  further  in  the  mass  of  new  secondaiy  dentine, 
the  tubular  structure  again  asserts  itself  more  strongly  :  this 
is  well  seen  in  the  specimen  figured. 


(')  Secondary  dentine  filling  up  one  of  the  cornua    of   the  puli^   cavity. 
From  a  human  molar  affected  by  caries. 


TEE   DENTAL    TISSUES. 


95 


CEJIEXTUM. 

The  cement  forms  a  coating  of  variable  thickness  over 
the  roots  of  the  teeth,  sometimes,  when  the  several  roots  are 
very  close  to  another,  or  the  cement  is  thickened  by  disease, 
uniting  the  several  roots  into  one. 

The  cement  is  ordinarily  said  to  be  absent  from  the 
crowns  of  the  teeth  of  man,  the  carnivora,  ifec,  and  to  com- 
mence by  a  thin  edge  just  at  the  neck  of  the  tooth,  over- 
lapping the  enamel  to  a  slight  extent ;  it  is,  in  the  healthy 
state,  thickest  in  the  interspaces  between  the  roots  of  molar 
or  bicuspid  teeth  :  it  is,  however,  often  thickened  at  the  end 


Fig.  51  ('). 


of  a  root  by  a  dental  exostosis.  In  compound  teeth,  the 
cementum  forms  the  connecting  substance  between  the  den- 
ticles (see  the  figures  of  the  tooth  of  the  Capybara,  the 
Elephant,  &c.),  and,  before  the  tooth  has  been  subject  to 
wear,  foi'ms  a  complete  investment  over  the  top  of  the 
crown.  The  cementum  also  covers  the  croAras  of  the  com- 
(')  Thick  laminated  cementum  from  tlie  root  of  a  human  tooth. 


A    MANUAL    OF   DENTAL    ANATOMY. 


plex-pattemed  crowns  of  the  teeth  of  ruminants ;  and,  in 
my  opinion,  is  present  in  a  nidimentary  condition  upon  the 
teeth  of  man,  &c.,  as  Nasmyth's  membrane.  The  cementum 
is  the  most  external  of  the  dental  tissues  :  a  fact  which  ne- 
cessarily follows  from  its  being  derived  more  or  less  directly 
from  the  tooth  follicle. 

Both  physically  and  chemically,  and  also  in  respect  of  the 
manner  of  its  development,  the  cementum  is  closely  allied 
to  bone.  It  consists  of  a  calcified  matrix  or  Viasal  substance, 
to  a  slight  extent  laminated,  and  lacuna?.  Vascular  canals 
corresponding  to  the  Haversian  canals  of  bone,  are  met  with, 
but  it  is  only  in  thick  cementum  that  they  exist ;  and,  in 
man,  perhaps  in  exostosis  more  often  than  in  the  thick 
healthy  tissue. 

The  matrix  is  a  calcified  substance,  whicli,  when  boiled 

Fig.  52  (  ). 


yields  gelatine,  and  if  decalcified  retains  its  form  and  struc- 
ture :  it  is,  in  fact,  practically  identical  with  the  matrix  of 
bone.  It  is  sometimes  apparently  structureless,  at  others 
finely  granvdar,  or  interspersed  with  small  globules. 

The  lacun?e  of  cementum  share  with  those  of  bone  the 
following  characters  :    in  dried  sections  they  are    irregular 


(')  Lacuna  of  cementum  which  communicates  with  the  terminations  of 
the  dentinal  tubes. 


THE   DENTAL    TISSUES.  97 

cavities,  elongated  in  the  direction  of  the  lamellse  of  the 
matrix,  and  furnished  with  a  large  number  of  processes. 
The  processes  of  the  lacunae  (known  as  canaliculi)  are  most 
abundantly  given  off  at  right  angles  to  the  lamellse  (see 
Fig.  51),  and,  again,  in  cementum,  are  more  abundantly- 
directed  towards  the  exterior  of  the  root  than  towards  the 
dentine.  The  lacunae  of  cementum  differ  from  those  of 
bone  in  being  far  more  variable  in  size,  in  form,  and  in  the 
excessive  number  and  length  of  their  canaliculi ;  in  this 
latter  respect  the  lacunae  of  the  cement  of  Cetacean  teeth  are 
very  remarkable. 

Many  of  the  lacunae  in  cementum  are  connected,  by  means 
of  their  canaliculi,  with  the  terminations  of  the  dentinal 
tubes  (Fig.  52) ;  they,  by  the  same  means,  freely  intercom- 
municate with  one  another,  while  others  of  their  processes 
are  directed  towards  the  surface,  which,  however,  in  most 
instances,  they  do  not  appear  to  actually  reach. 

The  lacunae  assume  all  sorts  of  peculiar  forms,  especially 
in  the  thicker  portion  of  the  cement. 

Hei-e  and  there  lacunae  are  to  be  found  which  are  fur- 
nished with  comparatively  short  processes,  and  are  contained 
within  well-defined  contours.  Sometimes  such  a  line  is  to 
be  seen  surrounding  a  single  lacuna,  sometimes  several 
lacunae  are  enclosed  within  it ;  lacunae  so  circumscribed  are 
called  "  encapsuled  lacunae,"  and  were  first  observed  by 
Gerber  in  the  cement  of  the  teeth  of  the  horse  (they  are 
specially  abundant  in  the  teeth  of  the  solidung\ilata).  By 
cautious  disintegration  of  the  cementum  in  acids  these  en- 
capsuled  lacunae  may  be  isolated ;  the  immediate  walls  of 
the  lacunae  and  canaliculi,  just  as  in  bone,  being  composed 
of  a  material  which  has  more  power  of  resisting  chemical 
re-agents  than  the  rest  of  the  matrix. 

The  enca:psiiled  lacunae  are  to  be  regarded  as  individual 
osteoblasts,  or  nests  of  osteoblasts,  with  a  common  connective 

H 


A    MANUAL    OF   DENTAL    ANATOMY. 


tissue  iuvestment,  which  have  to  some  extent  preserved  their 
individuality  during  calcification. 

In  the  fresh  condition  it  appears  probable  that  the  lacunas 
are  filled  up  by  soft  matrix,  which  shrinks  up,  and  so  leaves 
them  as  cavities  in  dined  sections.  It  can  hardly  as  yet  be 
said  that  the  question  of  the  contents  of  lacunse  has  been 
finally  settled,  though  the  researches  of  Bcidecker  and  Heitz- 
mann  have  gone  far  towards  doing  so. 

According  to  them  each  lacuna  contains  a  protoplasmic 
body,  which  they  term  the  cement  corpuscle,  with  a  central 
nucleus. 

This  nucleus  may  be  large  and  surrounded  by  but  little 
protoplasm,  or  it  may  be  small ;  or  there  may  be  many 
nuclei. 

The  cement  corpuscles  communicate  freely  with  one  another 
by  offshoots,  those  of  large  size  occupying  the  conspicuously 
visible  canaliculi  of  the  lacuna;,  whilst  the  finer  offshoots  are 
believed  by  them  to  form  a  delicate  network  through  the 
whole  basis  substance  or  matrix.  The  cement  corpuscles  near 
to  the  external  surface  give  off  numerous  offshoots  which 
communicate  with  protoplasmic  bodies  in  the  periosteum. 
By  this  means  the  cementum  can  remain  alive  even  when 
the  pulp  of  the  tooth  is  dead,  and  thus  the  tooth  is  in  no 
way  a  mere  foreign  body,  dead  and  inert. 

Like  bone,  cementum  is  also  sometimes  found  to  contain 
Shai'pey's  fibres ;  that  is  to  say,  rods  running  through  it  at 
right  angles  to  its  own  lamination,  and,  as  it  were,  perforat- 
ing it.  These  are  probably  calcified  bundles  of  connective 
tissue. 

Where  the  cementum  is  very  thin,  as,  for  instance,  where 
it  commences  at  the  neck  of  a  human  tooth,  it  is  to  all 
appearance  structureless,  and  does  not  contain  any  lacunse, 
and  therefore  no  protoplasmic  bodies :  nevertheless  lacunae 
may  be  sometimes  found  in  thin  cementum,  as,  for  example. 


THE   DENTAL    TISSUES. 


in  that  thiu  la3-er  which  invests  the  front  of  the  enamel  of 
the  rodent-like  tooth  of  a  wombat. 

The  cementum  at  the  neck  is  also  devoid  of  lamellae ;  it 
appears  to  be  built  up  by  direct  ossification  of  osteoblasts, 
the  prismatic  shape  of  which  may  be  traced  in  it  :  Bodecker 
describes  it  as  permeated  by  a  fine  but  abundant  network 
of  soft  living  matter.  The  larger  dentinal  tubes  fall  short 
of  the  boundary  line  at  the  neck,  but  a  fine  protoplasmic 
network  ci'osses  it.  Bodecker  states  that  it  has  a  covering 
of  epithelial  elements,  like  those  of  the  gum. 

The  outermost  layer  of  thick  cementum  is  a  glassy  film, 
denser  apparently  than  the  subjacent  portions,  and  quite 
devoid  of  lacunse ;  on  the  surface  it  is  slightly  nodular,  and 
might  be  described  as  built  up  of  an  infinite  number  of 
very  minute  and  perfectly  fused  globules ;  this  is,  in  fact, 
the  youngest  layer  of  cement,  and  is  closely  similar  to  that 
globular  formation  which  characterizes  dentine  at  an  early 
stage  of  its  development. 

The  cementum  is  veiy  closely,  indeed  inseparably,  con- 
nected with  the  dentine,  through  the  medium  of  the 
"  granular  "  layer  of  the  latter ;  the  fusion  of  the  two  tissues 
being  so  intimate,  that  it  is  often  diflicult  to  say  precisely 
at  what  point  the  one  may  be  said  to  have  merged  into  the 
other.  And  in  this  region  there  is  an  abundant  passage  of 
protoplasmic  filaments  across  from  the  one  to  the  other. 

Nasmyth's  membrane. — Under  the  names  of  Nasmyth's 
membrane,  enamel  cuticle,  or  persistent  dental  capsule,  a 
structure  is  described  about  which  much  difference  of 
opinion  has  been,  and  indeed  still  is,  expressed.  Over  the 
enamel  of  the  crown  of  a  hiunan  or  other  mammalian  tooth„ 
the  crown  of  which  is  not  coated  by  a  thick  layer  of  cemen- 
tum, there  is  an  exceedingly  thin  membrane,  the  existence 
of  which  can  only  be  demonstrated  by  the  use  of  acids, 
which  cause  it  to  become  detached  from  the  sui-face  of  the 
enamel.     When  thus  isolated  it  is  found  to  form  a  continu- 

H  2 


100  A    MANUAL    OF   DENTAL    ANATOMY. 


0118  transparent  sheet,  upon  -which,  by  staining  with  nitrate 
of  silver,  a  reticulated  pattern  may  be  brought  out,  as 
though  it  Avere  made  up  of  epithelial  cells.  The  inner 
surface  of  Nasmyth's  membrane  is,  however,  pitted  for  the 
reception  of  the  ends  of  the  enamel  prisms,  which  may  have 
something  to  do  with  this  reticulate  appearance.  It  is 
exceedingly  thin,  Kolliker  attributing  to  it  a  thickness  of 
only  one  twenty-thousandth  of  an  inch,  but,  nevertheless, 
it  is  very  indestructible,  resisting  the  action  of  strong  nitric 
or  hydrochloric  acid,  and  only  swelling  slightly  when  boiled 
in  caustic  potash.  Notwithstanding,  however,  that  it  resists 
the  action  of  chemicals,  it  is  not  so  hard  as  the  enamel,  and 
becomes  worn  off  tolerably  speedily,  so  that  to  see  it  well  a 
young  and  umvorn  tooth  should  be  selected. 

Observations  upon  the  presence  or  absence  of  Nasmyth's 

Fig.  53  ('V 


membrane  in  fish  and  reptiles  are  very  much  needed  ;  my 
own  recent   investigations   upon   the   development   of  the 

(•)  From  a  section  of  a  bicuspid  tooth  in  which  the  cemcntum,  c,  is  con- 
tinued over  the  outside  of  the  enamel,  a  ;  the  dentine  is  indicated  by  the 
letter  h. 


THE   DENTAL    TISSUES. 


teeth  in  these  classes  make  me  doubt  whether  the  d  priori 
conclusion  of  Waldeyer,  who  believes  that  the  cuticle  will 
be  found  on  all  teeth,  is  not  based  upon  an  interpretation  of 
its  nature  which  is  incon-ect. 

The  observation  of  Professor  Huxley,  who  believed  that 
he  found  it  upon  the  teeth  of  the  frog,  &c.,  may  be  suscep- 
tible of  another  explanation,  to  which  I  shall  have  to  I'ecur, 
merely  premising  here  that  its  presence  is  only  certain  in 
Primates,  Carnivora,  and  Insectivora. 

The  singular  power  of  resistance  to  re-agents  which 
characterises  it  proves  nothing  more  than  that  it  is  a  tissue, 
imperfectly  calcified,  on  the  border-land  of  calcification,  so 
to  speak,  since  similarly  resistant  structures  are  to  be  found 
lining  the  Haversian  canals,  the  dentinal  tubes,  the  surface 
of  developing  enamel,  the  lacunse,  &c. 

In  my  father's  opinion  (Dental  Surgery,  1859)  it  is  to  be 


Fir..  54  {'). 

"..^^ 

'^^"'^^^^^^M'T^iil 

^M'Mi 

\ 

/^.f  ?;'    '  / 

\/ 

,J:      >'':■     ■  '    "^    ,  , 

regarded  as  a  thin  covering  of  cementum,  and  I  have  given 
additional  evidence  in  support  of  the  view  in  a  paper 
refen-ed  to  abeady  in  the  list  of  works  which  heads  this 
chapter. 

It  now  and  then  happens  that  the  cementum  upon  a 
more  or  less  abnormal  tooth,  instead  of  ceasing  at  the  neck, 
(')  Encai>suled  lacuna  occupying  a  pit  in  the  enamel. 


102  A    MANUAL    OF   DENTAL    ANATOMY. 


is  continued  up  over  the  exterior  of  the  enamel.  This  has 
occurred  less  uncommonly  than  is  generally  imagined,  and 
the  accompanying  figure  represents  a  portion  of  the  crown 
of  such  a  tooth. 

If  the  section  be  made  of  the  grinding  surfaces  of  such 
teeth  as  present  rather  deep  fissures  in  these  situations, 
well  marked  and  unmistakeable  lacunal  cells,  or  encapsuled 
lacuna),  will  be  met  with  with  considerable  frequency.     Now 

Fig.  .55  ('). 


cl    . 


and  then  an  encapsuled  lacuna  may  be  found  occupying  a 
shallow  depression  in  the  enamel  which  it  just  fits,  but 
more  commonly  a  dozen  or  more  are  crowded  together  in  a 
pit  in  the  enamel,  where  they  are  usually  stained  of  a  brownish 

{})  Nasmyth's  membrane,  set  free  by  the  partial  solution  of  the  enamel. 
a.  Nasmyth's  membrane,  h.  Dentine,  d.  Mass  occupying  a  pit  in  the 
enamel,     e.  Enamel.     «'.  Torn  end  of  Nasmyth's  membrane. 


THE  DENTAL    TISSUES. 


colour.  The  occurrence  of  lacunse  in  these  situations  is  far 
from  rare  :  my  father's  collection  contains  more  than  a  dozen 
good  examples  of  them  in  these  positions. 

Nasmyth's  membrane,  thin  though  it  is  over  the  exterior 
of  the  enamel,  is  thickened  when  it  covers  over  a  pit  or 
fissure,  and  when  isolated  by  an  acid  is  seen  to  have  entirely 
filled  up  such  spots.     (Fig.  55). 

In  these  places,  then,  where  the  encapsuled  lacunse  ai'e  to 
be  found,  Nasmyth's  membrane  also  exists,  a  fact  which 
alone  would  lend  some  probability  to  the  view  that  it  is 
cementum. 

The  general  absence  of  lacuna;  in  Nasmyth's  membrane 
is  due  to  the  fact  that  it  is  not  thick  enough  to  contain  them; 
just  as  the  thinnest  layers  of  unquestionable  cementum  also 
are  without  lacunse. 

In  sections  of  an  unworn  bicuspid  which  was  treated  with 
acid  subsequently  to  its  having  been  ground  thin  and  placed 
upon  the  slide,  I  have  several  times  been  fortunate  enough 
to  get  a  view  of  the  membrane  in  situ  ;  it  then  appears  to 
be  continuous  with  an  exterior  layer  of  cementum,  which 
becomes  a  little  discoloured  by  the  acid  employed  to  detach 
Nasmyth's  membrane  from  the  enamel.  I  am  therefore 
inclined  to  regard  it  as  young  and  incomplete  cementum, 
and  to  consider  it  as  representing  (upon  the  human  tooth) 
the  thick  cementum  which  covers  the  crowns  of  the  teeth  of 
Hcrbivora  ]  and  I  am  very  glad  to  learn  from  my  friend 
Dr.  Magitot,  who  has  made  many  as  yet  unpublished  re- 
searches upon  this  subject,  that  he  entirely  concurs  in  this 
view,  which  has  also  the  support  of  Professor  Wedl. 

The  evidence  offered  that  Nasmyth's  membrane  is  cementum, 
although  strong-,  does  not  amount  to  absohite  proof  ;  it  is  therefore 
desirable  to  briefly  recapitulate  the  other  explanations  of  its  nature 
which  have  been  offered. 

Xasmyth,  who  fii-st  called  attention  to  its  existence,  regarded  it 
as  •'  persistent  dental  capsule  ;  "  a  view  of  its  nature  not  very  ma- 
terially differing  from  that  advocated  in  these  pages. 


104  A    MANUAL    OF    DENTAL    ANATOMY. 


Professor  Huxley  described  ib  as  being  identical  with  the  mein> 
brana  performativa  ;  that  is  to  say,  with  a  membrane  which  covered 
the  dentine  papilla  prior  to  the  occun-ence  of  calcification,  and 
which  afterwards  came  to  intervene  between  the  formed  enamel 
and  the  enamel  organ.  The  objections  to  the  acceptance  of  this, 
view  of  its  nature  are  so  inextricably  wrapped  up  with  other  ob- 
jections to  Professor  Huxley's  theory  of  the  development  of  the- 
teeth,  that  they  cannot  profitably  be  detailed  in  this  place  ;  it  will 
suffice  to  say,  that  evidence  and  the  weight  of  authority  alike  point. 
to  there  being  no  such  true  membrane  as  this  membrana  performa- 
tiva in  the  place  in  question. 

Waldeyer  holds  that  it  {\.c.,  Nasmyth's  membrane)  is  a  product  of 
a  part  of  the  enamel  organ.  After  the  completion  of  the  formation 
of  the  enamel  he  believes  that  the  cells  of  the  external  epithelium 
of  the  enamel  organ  become  applied  to  the  surface  of  the  enamel 
and  there  corn  ijird ;  in  this  way  he  accounts  for  its  resistance  ta 
reagents,  and  for  its  peculiar  smell  when  it  is  burnt. 

Its  extreme  thinness,  so  far  as  it  goes,  is  an  objection  to  this 
supposition  :  a  more  weighty  argument  against  it  is  the  absence  of 
analogy  for  such  a  peculiar  change,  by  which  one  portion  of  the 
same  organ  is  calcified,  and  the  rest  cornified  ;  and  again,  what  be- 
comes of  these  cells  in  those  teeth  in  which  cementum  is  deposited 
in  bulk  over  the  surface  of  the  enamel .'  According  to  the  statement. 
of  Dr.  Magitot,  the  layer  of  cells  in  question  (external  epithelium 
of  the  enamel  organ)  is  atrophied  before  the  time  of  the  completion 
of  the  enamel ;  a  fact  which,  if  confinned,  is  fatal  to  Waldeyer's 
explanation.  And  Dr.  Magitot,  in  his  most  recent  paper  on  the 
subject  (Joiu-nal  de  Tanatomie,  &c.,  1881),  gives  his  adherence  to 
the  view  that  it  is  cementum. 

KoUiker,  who  dissents  strongly  from  the  views  of  Waldeyer,  and 
admits  some  uncertainty  as  to  its  nature,  provisionally  regards  it  as 
a  continuous  and  structureless  layer  furnished  by  the  enamel  cells, 
after  their  work  of  forming  the  fibrous  enamel  was  complete  ;  a 
sort  of  varnish  over  the  surface,  as  it  were. 

This  would  not  account  for  the  occui-rence  of  lacunaj  in  it. 


The  Tooth  Pulp. 

The  Tooth  Pulp,  occupying  the  central  chamber,  or  pulp 
cavity,  is  the  formative  organ  of  the  tooth,  and  consequently 
varies  to  some  extent  in  its  anatomical  character  according 
to  its  age.  As  well  as  being  what  remains  of  a  formative 
organ,  it  is  the  vascular  and  nervous  source  of  supply 
whence  the  dentine  mainly  derives  its  vitality. 


THE   DENTAL    TISSUES. 


The  pulp  may  be  described  as  being  made  up  of  a  mucoid, 
gelatinous  matrix,  containing  cells  in  abundance,  which  are 
especially  numerous  near  to  its  periphery.  In  it  some 
fibrous  connective  tissue  is  discoverable,  though  this  is  not 
abundant  until  the  period  of  degeneration  has  set  in.  Nerves 
and  vessels  also  ramify  abundantly  in  it. 

The  cellular  elements  of  the  pvdp  are  arranged,  as  seen 
in  transverse  sections,  in  a  direction  radiating  outwards  from 
the  centre ;  this  is  most  marked  in  the  highly  specialised 
layer  of  cells  which  form  the  surface  of  the  pulp,  and  are 
termed  odontoblasts. 

The  odontoblast  layer,  sometimes  called  the  membrana 
ehoris,  because  it  usually  adheres  more  strongly  to  the 
dentine  than  to  the  rest  of  the  pulp,  and  is  therefore  often 
left  behind  upon  the  dentine  when  the  pulp  is  torn  away, 
consists  of  a  single  row  of  large  elongated  cells,  of  darkish 
granular  appearance,  with  a  large  and  conspicuous  nucleus 
near  to  the  end  farthest  from  the  dentine. 

The  sharp  contours  which  the  odontoblasts  possess  iu 
pulps  which  have  been  acted  on  by  chromic  acid,  alcohol, 
or  even  water,  are  absent  in  the  perfectly  fresh  and  unaltered 
condition,  and  it  is  believed  that  they  have  no  special  in- 
vesting membrane.  They  are  furnished  with  three  sets  of 
processes.  The  dentinal  process  (which  is  equivalent  to  the 
dentinal  fibre)  enters  the  canal  in  the  dentine,  and  the  in- 
dividual odontoblast  may  be  furnished  with  several  dentinal 
processes.  By  means  of  lateral  processes  the  cells  communi- 
cate with  those  on  either  side  of  them,  and  by  means  of 
their  pidp  processes  with  cells  lying  more  deeply;  these 
deeper  cells  again  are  to  some  extent  intermediate  in  size 
between  the  odontoblasts  and  the  internal  cells  of  the  pulp. 
The  membrana  eboris  covers  the  surface  of  the  pulp  like 
an  epithelium.  The  odontoblasts  vary  much  in  form  at 
different  periods  ;  in  the  youngest  puljis,  prior  to  the  forma- 
tion  of  dentine,    they    are    roundish,   or  rather   pyriform  ; 


106  A    MANUAL    OF   DENTAL    ANATOMY. 

during  the  period  of  their  greatest  functional  activity  the 
end  directed  towards  the  dentine  is  squai'ish,  though  tapering 
to  a  slight  extent  into  the  dentinal  process ;  while  in  old 
age  they  become  comparatively  inconspicxious,  and  assume 
a  rounded  or  ovoid  shape.  The  general  matrix  of  the  pulp, 
as  has  been  before  noted,  is  of  firm,  gelatinous  consistency : 
it  is  a  little  more  dense  upon  the  surface,  whence  has 
perhaps  arisen  the  erroneous  idea  that  the  pulp  is  bounded 
by  a  definite  membrane. 

The  vessels  of  the  pulp  are  very  numerous  ;  three  or 
more  arteries  enter  at  the  apical  foramen,  and  breaking  up 
into  branches  which  are  at  first  parallel  with  the  long  axis 
of  the  pulp,  finally  form  a  capillary  plexus  immediately 
beneath  the  cells  of  the  membrana  eboris. 

No  lymphatics  are  known  to  occur  in  the  tooth  pulp. 

The  nerves  enter  usually  by  one  largish  trunk  and  three 
or  four  minute  ones  :  after  pursuing  a  parallel  course,  and 
giving  off"  some  branches  which  anastomose  but  little,  in  the 
expanded  portion  of  the  pulp  they  form  a  rich  plexus  beneath 
the  membrana  eboris,  as  has  been  described  by  Raschkow 
and  many  subsequent  writers. 

But  here  our  exact  knowledge  ends,  for  the  nature  of  the 
terminations  of  the  nerve  fibres  in  the  pulp  is  not  with 
■certainty  known :  the  primitive  fibrils,  which  are  extra- 
ordinarily abiindant  near  to  the  surface  of  the  pulp,  often 
form  meshes,  but  this  does  not  appear  to  be  their  real 
termination. 

Boll,  as  has  been  mentioned  at  a  previous  page,  investi- 
gated this  point,  and  found  that  if  a  pulp  be  treated  for  an 
hour  with  very  dilute  chromic  acid  solution,  an  immense 
number  of  fine  non-meduUated  nerve  fibres,  which  he  suc- 
ceeded in  tracing  into  continuity  with  the  larger  meduUated 
fibres,  may  be  discerned  near  to  the  surface  of  the  pulp. 
The  ultimate  destination  of  these  nerve  fibres  is  un- 
certain ;  but  he  has  seen  them  passing  through  the  mem- 


THE    DENTAL    TISSUES.  T07 


brana  eboris,  and  taking  a  direction  parallel  to  that  of  the 
dentinal  fibrils  in  such  numbers  that  he  infers  that  they 
have  been  pulled  out  from  the  canals  of  the  dentine.  Still, 
whatever  may  be  the  probabilities  of  the  case,  he  has  not 
seen  a  nerve  fibre  definitely  to  pass  into  a  dentinal  canal, 
nor  has  any  other  observer  been  more  fortunate. 

Boll's  observations  have  not  however  been  fully  confirmed 
by  any  subsequent  worker  in  the  field,  nor  have  they  been 
definitely  controverted  until  Magitot  recently  stated  that  he 
had  fully  satisfied  himself  that  tlie  nerves  become  continuous 
with  the  branched  somewhat  stellate  cells  which  form  a  layer 
beneath  the  odontoblasts,  and  through  the  medium  of  these 
cells,  with  the  odontoblasts  themselves.     (See  Fig.  32.) 

If  this  view  of  their  relation  to  the  nerves  be  correct  the 
sensitiveness  of  the  dentine  would  be  fully  accounted  for 
without  the  necessity  for  the  supposition  that  actual  nerve 
fibres  enter  it,  for  the  dentinal  fibrils  would  be  in  a 
measure  themselves  prolongations  of  the  nerves. 

It  has  already  been  mentioned  that  the  pulp  undergoes 
alterations  in  advanced  age,  its  diminution  in  size  by  its 
progressive  calcification  and  the  addition  thus  made  to  the 
walls  of  the  pulp  cavity  being  the  most  conspicuous  change 
which  occurs.  In  pulps  w^hich  have  undergone  a  little 
further  degeneration,  the  odontoblast  layer  becomes  atro- 
phied; fibrillar  connective  tissue  becomes  more  abundant, 
coincidently  with,  the  diminution  in  the  quantity  of  the  cel- 
lular elements.  Finally,  the  capillary  system  becomes 
obliterated  by  the  occurrence  of  thrombosis  in  the  larger 
vessels,  the  nerves  undergo  fatty  degeneration,  and  the  pulp 
becomes  reduced  to  a  shrivelled,  unvascular,  insensitive 
mass.  These  changes  may  go  on  without  leading  to  actual 
putrefactive  decomposition  of  the  pulp,  and  are  hence  not 
attended  by  aveolar  abscess  ;  but  a  tooth  in  wdiich  the  pulp 
has  undergone  senile  atrophy  is  seldom  fast  in  its  socket. 

The  pulps  of  the  teeth  of  some  animals  become  eventually 


108  A    MANUAL    OF   DENTAL    ANATOMY. 

entirely  converted  into  secondary  dentine,  but  it  would  seem 
to  be  very  generally  the  case  that  those  teeth  which  exercise 
very  active  functions  and  last  tliroughout  the  life  of  the 
creature  retain  their  j)ulp  in  an  active  and  vascular  condition. 


The  Gum. 

The  gum  is  continuous  with  the  mucous  membrane  of 
the  inside  of  the  lips,  of  the  floor  of  the  mouth,  and  of  the 
palate,  and  differs  from  it  principally  by  its  greater  density. 
Its  hardness  is  in  part  due  to  the  abundant  tendinous  fasci- 
culi which  it  itself  contains,  in  part  to  its  being  closely 
bound  down  to  the  bone  by  the  blending  of  the  dense  fibrous 
fasciculi  of  the  periosteum  with  its  own.  The  fasciculi 
springing  from  the  periosteum  spread  out  in  fan-like  shape 
as  they  approach  the  epithelial  surface.  There  is  thus  no 
very  sharp  line  of  demarcation  between  the  gum  and  the 
periosteum  when  these  are  seen  in  section  in  situ. 

The  gum  is  beset  with  rather  large,  broad-based  papilla, 
which  are  sometimes  single,  sometimes  compound ;  the 
epithelium  is  composed  of  laminae  of  tesselated  cells,  much 
flattened  near  to  the  surface ;  but  cylindrical  cells  form  the 
deepest  layer  of  the  epithelium,  the  rete  malpighi. 

Small  round  aggi-egations  of  pavement  epithelium  are 
met  with  at  a  little  depth,  or  even  bedded  in  the  smface  ; 
these,  the  "glands"  of  Sei-res,  have  no  known  significance. 
In  the  neighbourhood  of  developing  tooth-sacs  ejiithelial 
aggregations  of  similar  appearance  are  to  be  met  with,  and 
in  such  spots  are  remains  of  the  neck  of  the  enamel  organ 
(cf  page  137),  which  has  undergone  this  curious  change 
subsequently  to  the  completion  of  its  original  function. 
The  gums  are  rich  in  vessels,  but  remarkably  scantily  sup- 
plied with  nerves. 


THE   DENTAL    TISSUES. 


At  the  necks  of  the  teeth  tlie  gum  becomes  continuous 
with  the  periosteum  of  the  internal  surface  of  the  alveoli, 
into  which  it  passes  without  any  line  of  demarcation. 


The  Alveolo-Dextal  Membrane. 

The  Alveolo-dental  'Periosteum,  or  Eoot  membrane,  is  a 
connective  tissue  of  moderate  density,  devoid  of  elastic 
fibres,  and  richly  supplied  with  nerves  and  vessels. 

It  is  thicker  near  to  the  neck  of  the  tooth,  where  it 
passes  by  imperceptible  gradations  into  the  gum  and  peri- 
osteum of  the  alveolar  process,  and  near  to  the  apex  of  the 
root.  The  general  direction  of  the  fibres  is  transverse ;  that 
is  to  say,  they  n;n  across  from  the  alveolus  to  the  cemen- 
tum,  without  break  of  continuity,  as  do  also  many  capillary 
vessels ;  a  mere  inspection  of  the  connective  tissue  bundles, 
as  seen  in  a  transverse  section  of  a  decalcified  tooth  in  its 
socket,  will  suffice  to  demonstrate  that  there  is  but  a  single 
■"membrane,"  and  that  no  such  thing  as  a  membrane  proper 
to  the  root  and  another  proper  to  the  alveolus  can  be  dis- 
tinguished ;  and  the  study  of  its  development  alike  proves 
that  the  soft  tissue  investing  the  root,  and  that  lining  the 
socket,  are  one  and  the  same  thing :  that  there  is  but  one 
•"  membrane,"  namely,  the  alveolo-dental  periosteum. 

At  that  part  which  is  nearest  to  the  bone  the  fibres  are 
grouped  together  into  conspicuous  bundles;  it  is,  in  fact, 
much  like  any  ordinary  fibrous  membrane.  On  its  inner 
aspect,  where  it  becomes  continuous  with  the  cementum,  it 
consists  of  a  fine  network  of  interlacing  bands,  many  of 
which  lose  themselves  in  the  surface  of  the  cementum. 

But  although  there  is  a  marked  difference  in  histological 
character  between  the  extreme  parts  of  the  membrane,  yet 
the  markedly  fibrous  elements  of  the  outer  blend  and  pass 


110  A    MANUAL    OF   DENTAL    ANATOMY. 

insensibly  into  the  bands  of  the  fine  network  of  the  inner 
part,  and  there  is  no  break  of  continuity  whatever. 

At  tlie  surface  of  the  cementum  it  is  more  richly  cellular, 


Fig.  56  ('). 


>*w.ftV 


and  here  occur  abundantly  large  soft  nucleated  plasm 
masses,  which  are  the  osteoblasts  concerned  in  making 
cementum,  and  which  by  their  offshoots  communicate  with 
plasm  masses  imprisoned  within  the  cementum. 

Q)  Portion  of  tlie  side  of  the  root  of  a  tooth,  the  gum  and  alveolar 
dental  membrane,  and  the  edge  of  the  bone  of  the  alveolus. 

A  band  of  fibres  is  seen  passing  over  the  surface  of  the  alveolus  and 
dividing,  some  to  pass  upwards  into  the  gum,  others  to  pass  more  directly 
across  to  the  cementum.  Numerous  orifices  of  vessels  cut  across  trans- 
versely are  seen  between  the  tooth  and  the  bone. 


THE   DENTAL    TISSUES.  Ill 

I  have  never  seen  the  fibres,  whether  in  longitudiual  or 
in  transverse  sections,  pass  straight  in  the  shortest  possible 
line  from  the  bone  to  the  cementum,  but  they  invariably 
pursue  an  oblique  course,  which  probably  serves  to  allow  for 
slight  mobility  of  the  tooth  without  the  fibres  being  stretched 
or  torn. 

The  vascular  supply  of  the  root  membrane  is,  according 
to  Wedl,  derived  from  three  sources ;  the  gums,  the  vessels 
of  the  bone,  and  the  vessels  destined  for  the  pulp  of  the 
tooth,  the  last  being  the  most  important. 

The  nerve  supply  also  is  largely  derived  from  the  dental 
nerves  running  to  the  dental  pulps ;  other  filaments  come 
from  the  inter-alveolar  canals  (canals  in  the  bone,  contain- 
ing nerves  and  vessels,  which  are  situated  in  the  septa 
separating  the  alveoli  of  contiguous  teeth). 

It  should  be  borne  in  mind  that  the  tooth  pulp  and  the 
tissue  which  becomes  the  root  membrane  have  sprung  from 
the  same  source,  and  were  once  continuous  over  the  whole 
base  of  the  pulp.  A  recognition  of  this  fact  makes  it  easier 
to  realise  how  it  comes  about  that  their  vascular  and  nervous 
supplies  are  so  nearly  identical. 

The  human  tooth  is,  accepting  as  correct  the  researches 
of  Bodecker,  which  appear  in  every  way  deserving  of  credence, 
connected  with  the  living  organism  veiy  intimately,  even 
though  its  special  tissues  are  extra-vascular. 

For  blood  vessels  and  nerves  enter  the  tooth  pulp  in 
abundance ;  the  dentine  is  organically  connected  with  the 
pulp  by  the  dentinal  fibrils ;  these  are  connected  with  the 
soft  cement  corpuscles,  which  again  are  brought  by  their 
processes  into  intimate  relation  with  similar  bodies  in  the 
highly  vascular  periosteum. 

So  that  between  pulp  inside,  and  periosteum  outside,  there 
is  a  continuous  chain  of  living  plasm. 


112  A    MANUAL    OF   DENTAL    ANATOMY. 


K(»LLIKER.     Gewebelehre. 

Manual  of  Histology,  annotated  by  Messrs.  Busk  and 
Huxley,  1853. 
Waldeykr.     Strieker's  Histology.     1870. 
Prey.     Manual  of  Histology.     1874. 
Owen.     Odontography. 

CzERMAK.     Zeitschrift  f.  Wiss.  Zoologie.     ISoO. 
Neumann.     Zur  Kentniss  der  Xoi-malen  Zahngeweber.     1 8o3. 
Boll.     Untersuchungen  iiber  die  Zahnpulpa.    Archiv.  f.  Mikros. 

Anatom.     18(;8. 
Klein.    Atlas  of  Histology.     1 880. 
Salter.     Dental  Pathology.     1874. 
Tomes,  J.     Lectures  on  Dental  Physiology  and  Surgery.     1848. 

On  the  Dental  Tissues  of   Eodentia  and  Marsupialia. 

Philos.  Transac.  1840,  1850. 
On  the  Presence  of  Soft  Fibrils  in  Dentine.     Philos. 
Transac.  1853. 
Tomes,  Charles  S.     On  Vascular  Dentine.     Phil.  Trans.     1 878. 

On  the  Implantation  of  Teeth.   Proc.  Odontol. 

Soc.  1874—1876. 
On  Nasmyth's  Membrane.  Quart.  Jour.  Micros. 
Science,  1872. 
Magitot  et  Le«ros.    Journal  de  I'anatomie  de  M.   Ch.   Robin. 

1881, 
Petzius.     Mikrosk.  Undersok,  &c.     Stockholm.  18.S7,  and  Transla- 
tion in  Nasmyth  on  the  Teeth,  183II. 
Kasmyth.     On  the  Teeth.     1 839. 

Hertwig.     Ueber  der  Bau  der   Placoidschuppen  Jenaische  Zeit- 
schrift, B.  viii. 
Von  Boas.     Zahne  der  Scaroiden,  Zeits.  f.  Wiss.  Zoologie,  B.  xxxii. 
BoDECKER.     Dental  Cosmos.     1878. 

Lankester.  Ray.     On  the  Teeth  of  Micropteron.     Quart.  Journal 
Micros.  Science.  1857. 


CHAPTER  IV. 

THE  DEVELOPMENT  OP  THE  TEETH. 

The  development  of  the  teeth  is  a  process  which,  while 
subject  to  modifications  in  the  different  groups  of  verte- 
brates, retains  nevertheless  in  all  certain  essential  charac- 
ters, so  that  it  becomes  possible  to  embody  its  main  features 
in  a  general  account. 

Prior  to  the  commencement  of  any  calcification  there  is 
always  a  special  disposition  of  the  soft  tissues  at  the  spot 
where  a  tooth  is  destined  to  be  formed  ;  and  the  name  of 
•"  tooth  germ  "  is  given  to  those  portions  of  the  soft  tissue 
which  are  thus  specially  arranged.  All,  or  a  part  only,  of 
the  soft  stmctures  making  up  a  tooth  germ,  become  con- 
verted into  the  dental  tissues  by  a  deposition  of  salts  of  lime 
within  their  own  substance,  so  that  an  actual  conversion  of 
at  least  some  portions  of  the  tooth  germ  into  tooth  takes 
place.  The  tooth  is  not  secreted  or  excreted  by  the  tooth 
germ,  but  an  actual  metamorphosis  of  the  latter  takes 
place.  The  details  of  this  conversion  can  be  better  dis- 
cussed at  a  later  i)age  ;  for  the  present  it  -will  suffice  to  say 
that  the  three  principal  tissues,  namely,  dentine,  enamel 
and  cementum  are  formed  from  distinct  parts  of  the  tool  It 
germ,  and  that  we  are  hence  accustomed  to  speak  of  the 
enamel  germ  and  the  dentine  germ ;  the  existence  of  a 
special  cement  germ  is  asserted  by  Magitot,  but  as  yet  his 
descriptions  await  confirmation. 

In  many  anatomical  works  which  the  student  may  have 
occasion  to  consult,  the  process  of  tooth  development  may 


114  A    MANUAL    OF    DENTAL    ANATOMY. 

still  be  found  to  be  divided  into  periods,  under  the  names 
of  "  papillary,"  "  follicular,"  "and  "  eruptive  "  stages. 

These  stages  are  based  upon  a  false  conception,  upon 
theories  now  known  to  be  incon-ect,  and  may  advantageously 
be  absolutely  abandoned.  The  account  of  the  development, 
of  the  teeth  given  in  the  following  pages  (based  in  the 
case  of  man  and  mammals  upon  the  researches  of  Kulliker, 
Thiersch,  and  Waldeyer  ;  in  the  case  of  reptiles  and  fishes,, 
upon  those  of  Huxley  and  Santi  Sirena,  and  upon  Hertwig's 
and  my  own),  will  be  found  to  conflict  with  the  accounts 
published  by  a  deservedly  great  authority,  Professor  Owen. 
I  cannot  reconcile  these  discrepancies,  except  upon  the  as- 
sumption that  modern  methods  of  research  have  disclosed 
facts  heretofore  not  demonstrable ;  yet  twenty  years  ago. 
Professor  Huxley  demonstrated  in  a  remarkable  paper  the 
incorrectness  of  certain  of  the  theories  then  promulgated. 
Of  the  general  accuracy  of  the  following  description  I  am 
however  fully  satisfied,  and  most  of  the  facts  may  be  easily 
verified  by  any  one  desirous  of  so  doing. 

True  tooth  gei-ms  are  never  formed  quite  upon  the  surface  ('),. 
but  are  always  situated  at  a  little  distance  beneath  it,  lying  in 
some  creatures  at  a  considerable  depth.  Every  known  tooth 
germ  consists  in  the  first  instance  of  two  portions,  and  two 
only,  the  enamel  germ  and  the  dentine  germ ;  and  these  are 
derived  from  distinct  sources,  the  former  being  a  special 
development  from  the  epithelium  of  the  mouth,  the  latter 
from  the  more  deeply  lying  parts  of  the  mucous  membrane. 
Other  things,  such  as  a  tooth  capsule,  may  be  subsequently 
and  secondarily  formed,  but  in  the  first  instance,  every 
tooth  germ  consists  of  an  enamel  germ  and  a  dentine  germ 
only,  and  the  simplest  tooth  genus  never  develop  any  addi- 
tional parts.  The  existence  of  an  enamel  organ  in  an  early 
stage  is  therefore  perfectly  independent  of  any  subsequent 

(')  The  placoid  scales  of  embiyonic  sliarks  are,  however,  formed  on  the 
surface,  and  the  "germs"  covered  in  by  epithelium  only  (Hertwig). 


TEE   DEVELOrMEXT    OF    THE    TEETH.  115 

formation  of  enamel  by  its  own  conversiou  into  a  calcified 
tissue,  for  I  have  shown  that  it  is  to  be  found  in  the  germs 
of  teeth  which  have  no  enamel ;  in  fact,  in  all  tooth  germs 
"whatever. 

That  part  of  the  tooth  germ  destined  to  become  dentine 
is  often  called  the  dentine  pajDilla,  having  acquired  this 
name  from  its  papilliform  shape ;  and  in  a  certain  sense  it 
is  true  that  the  enamel  organ  is  the  epithelium  of  the  den- 
tine papilla.  Yet,  although  not  absolutely  untrue,  such  an 
expression  might  mislead  by  implying  that  the  enamel 
organ  is  a  secondary  development,  whereas  its  appearance 
is  contemporaneous  with,  if  not  antecedent  to,  that  of  the 
dentine  germ.  The  most  general  account  that  I  am  able 
to  give  of  the  process  is,  that  the  deeper  layer  of  the  oral 
epithelium  sends  down  into  the  subjacent  tissue  a  process, 
the  shape  and  structure  of  which  is,  in  most  animals,  dis- 
tinguishable and  characteristic  before  the  dentine  gei'm  has 
taken  any  definite  form.  This  process  enlarges  at  its  end, 
and,  as  seen  in  section,  becomes  divaricated,  so  that  it  bears 
some  resemblance  to  an  inverted  letter  Y ;  or  it  might  more 
truthfully  be  compared  to  a  bell  jar  with  a  handle ;  this 
constitutes  the  early  stage  of  an  enamel  germ  (see  Fig.  G-i), 
while  beneath  it  in  the  mucous  tissue,  the  dentine  gerai 
assumes  its  papilliform  shape.  The  details  of  the  process 
varying  in  different  creatures,  I  will  at  once  proceed  to  the 
description  of  the  development  of  teeth  in  the  various 
groups. 

In  Elasmobrancli  Fishes. — If  a  transverse  section 
through  the  jaw  of  a  dog-fish  (Scyllium  canieula)  be  ex- 
amined, we  shall  find  that  the  forming  teeth  lie  upon  the 
inside  of  the  semi-ossified  jaw-bones,  the  youngest  being  at 
the  bottom  (Fig,  57) ;  progi-essing  upwards,  each  tooth  is 
more  fully  calcified  till,  on  passing  over  the  border  of  the 
jaw,  we  come  to  those  teeth  whose  period  of  greatest  useful- 
ness is  passed  and  which  arc  about  to  be  cast  off  in  the 

I  2 


116  A    MANUAL    OF  DENTAL    ANATOMY. 

course  of  that  slow  rotation  of  the  whole  tooth-bearing 
mucous  membrane  over  the  border  of  the  jaw,  which  is 
constantly  going  on. 

In  the  section  figured  there  are  four  teeth  advanced  in 
calcification,  while  beneath  them  are  four  tooth  germs  in 
earlier  stages ;  of  the  former  two  only  are  fully  protruded 
throixgh  the  epithelium,  the  third  being  in  part  covered 
in  ;  the  remaining  teeth  are  altogether  beneath  the  surface 
of  the  epithelium,  and  therefore  shut  off  from  the  cavity  of 
the  mouth,  if  the  soft  parts  be  all  in  situ. 

All  the  teeth  not  fully  calcified  are  covered  in  and  pro- 
tected by  a  reflexion  upwards  of  the  mucous  membrane  (c  in 
the  figure),  which  serves  to  protect  them  during  their  calci- 
fication. 


But  although  this  may  be  termed  a  fold  reflected  upwards,  it  is 
not,  as  was  supposed  by  Professor  Owen,  a  free  flai^,  detached  from 
the  opposite  surface  on  which  the  teeth  are  developing ;  there  is  no 
deep  open  fissure  or  pouch  running  round  inside  the  jaw,  as  would 
in  that  case  exist,  and  the  epithelium  does  not  pass  down  on  the 
one  side  to  the  bottom  of  such  fissure,  and  then  ascend  upon  the 
other  as  a  distinct  layer.  Although  the  fold  is  very  easily  torn 
away  from  the  tooth  germs  which  it  covers  in,  yet  in  the  natural 
condition  it  is  attached,  and  there  is  no  breach  of  surface  ;  the 
epithelium  passing  across  from  the  jaw  to  cover  it  is  well  seen  in 
the  figure,  in  which  the  epithelial  layer  is  represented  as  broken 
just  at  the  point  (between  the  third  and  fourth  teeth)  where  it 
leaves  the  jaw  to  cross  over  on  to  the  surface  of  the  flap. 

The  conditions  met  with  in  the  Elasmobrauch  fishes  are 
peculiarly  favourable  for  the  determination  of  the  homolo- 
gies of  the  several  parts  of  the  tooth  germ  and  of  the  formed 
tooth  (').  At  the  base  of  the  jaw,  where  the  youngest 
tooth  germs  are  to  be  found,  the  tissue  whence  the  dentine 
papillye  arise  blends  insensibly  with  that  making  up  the 
substance  of  the  thecal  fold  on  the  one  hand,  and  on  the 

(})  Compare  the  description  of  the  placoid  dermal  spine  (page  2). 


THE   DEVELOrMEXT    OF    THE    TEETH. 


117 


other,  with  that  clothing  the  convexity  of  the  jaw  and  giving 
attachment  to  the  teeth. 

No  sharp  line  of  demarcation  at  any  time  marlcs  off  the 
base  of  the  dentine  papilla  from  the  tissue  which  surrounds 
it,  and  from  which  it  springs  up,  as  would  be  the  case  in 
mammalian  or  reptilian  tooth  germs ;  all  that  can  be  said 
is,  that  the  dentine  e;erms  are  cellular,  the  cells  being  large 


Fig.  57  ('). 


z.-^^ 


and  rounded,  while  in  the  rest  of  the  mucous  membrane 
the  fibrillar  elements  preponderate,  so  that  it  passes  by 
imperceptible  gradations  into  the  densely  fibrous  gum,  found 
on  the  exposed  border  of  the  jaw. 


(')  Trausvei'se  section  of  lowei"  jaw  of  a  Dog-fish.  a.  Oral  epithelium. 
h.  Oral  epithelium  jjassing  on  to  flap.  c.  Protecting  flap  of  mucous  mem- 
brane (thecal  fold),  d.  Youngest  dentine  pulp.  e.  Youngest  enamel 
organ.     /.   Tooth  about  to  be  shed.     (j.   Calcified  crust  of  jaw. 


118  A    MANUAL    OF  DENTAL   ANATOMY. 


The  dentine  germs,  and  consequently  the  dentine,  are 
indisputably  derived  from  the  connective  tissue  of  the 
mucous  membrane  immediately  subjacent  to  the  epithe- 
lium, nor  can  it  be  doubted  that  the  enamel  organs  are 
simply  the  modified  epithelium  of  that  same  mucous 
membrane. 

Of  course  there  is  nothing  new  in  this  conclusion,  which 
had  been  already  arrived  at  by  the  study  of  other  creatures, 
but  the  sharks  happen  to  demonstrate  it  with  more  clear- 
ness than  those  other  animals  in  whom  the  original  nature 
of  the  process  is  more  or  less  masked  by  the  introduction  of 
further  complexities. 

Hence  it  is  worth  while  to  study  carefully  the  relations 
of  the  epithelium  constituting  the  enamel  organs  with  that 
of  the  surface  of  the  mouth.  As  has  been  already  mentioned, 
in  the  normal  condition  of  the  part  there  is  no  deep  fissure 
on  the  inner  side  of  the  jaw,  but  the  epithelium  passes 
across  (from  the  interspace  between  the  third  and  fourth 
teeth  in  the  figure)  on  to  the  protecting  fold  of  mucous 
membrane  (c  in  fig.)  But  although  the  epithelium  is  re- 
flected across  on  to  the  thecal  fold,  it  is  also  continued 
downwards  along  the  inner  side  of  the  developing  teeth  and 
tooth  germs,  giving  to  each  a  complete  investment,  and 
filling  up  the  whole  interval  between  the  tooth  germ  and  the 
thecal  fold.  The  epithelium  in  this  situation  does  not,  then, 
consist  simply  of  one  layer  going  down  on  the  one  side  and 
covering  the  tooth  germs,  and  then  reflected  up  at  the 
bottom  to  coat  the  inner  side  of  the  thecal  fold,  but  it  is  so 
arranged  as  to  have  relation  only  to  the  tooth  germs ;  it  is 
termed  '•'  enamel  organs  "  because  over  the  tooth  germs  these 
epithelial  cells  assume  a  marked  columnar  character,  and 
are  very  different  in  appearance  from  the  epithelium  else- 
where. 

The  terminal  portion  of  this  epithelium,  or,  in  other 
words,  the  youngest  enamel  germ,  forms  a  bell-like  cap  over 


TEE  DEVELOPMENT    OF    THE    TEETH.  119 

the  eminence  of  mucous  membrane  connective  tissue  which 
constitutes  the  earliest  dentine  germ,  and  in  section  is  of 
the  form  shown  in  the  figure.  The  surface  next  to  the 
dentine  papilla  consists  of  elongated  columnar  cells,  with 
nuclei  near  to  their  attached  extremities,  while  the  rest  of 
its  substance  is  made  up  of  much  smaller  cells,  some  of 
which  have  inosciilating  processes,  so  that  they  constitute 
a  sort  of  finely  cellular  connective  tissue,  very  difi'erent  in 
appearance  from  anything  met  with  in  mammalian  enamel 
organs.  It  is  sufficiently  consistent  to  keep  up  the  con- 
tinuity of  all  the  enamel  organs,  even  when  displaced  in 
cutting  sections,  so  that  the  whole  might  be  described  as 
forming  one  composite  enamel  organ.  The  columnar  cells 
already  alluded  to  invest  the  whole  siu"face  which  is 
directed  towards  the  forming  teeth,  but  they  atrophy  some- 
what in  the  interspaces  of  the  tooth  germs. 

Before  proceeding  further  in  the  description  of  the  deve- 
lopment of  the  tooth  germs,  it  will  be  well  to  refer  to  a 
somewhat  earlier  stage  in  the  growth  of  the  Dog-fish,  in 
which  the  relation  subsisting  between  the  teeth  and  the 
dermal  spines  is  still  well  seen. 

On  the  lower  jaw  of  the  young  dog-fish  there  is  no  lip ; 
hence,  as  is  seen  in  the  figiire,  the  spines  which  clothe  the 
skin  come  close  to  the  dentigerous  surface  of  the  jaw. 

Although  there  are  differences  in  form  and  size,  a  glance 
at  the  figure  will  demonstrate  the  homological  identity  of  the 
teeth  and  the  dermal  spines.  As  the  dog-fish  increases  in 
size,  this  continuity  of  the  teeth  with  the  dermal  spines  on 
the  outside  of  the  head  becomes  interrupted  by  an  extension 
of  the  skin  to  form  a  lip ;  this  happens  earlier  in  the  upper 
jaw  than  in  the  lower,  and  at  first  the  spines  are  continued 
over  the  edge  and  the  inside  of  the  newly  formed  lip — from 
these  situations,  however,  they  soon  disappear.  In  structure, 
the  teeth  and  the  dermal  spines  are,  in  many  species,  very 
closely  similar ;  the  latter  are,  however,  much  less  often 


120 


A    MANUAL    OF   DENTAL    ANATOMY. 


shed  and  reproduced,  so  that  it  is  less  easy  to  find  them  in 
all  stages  of  their  growth  ;  I  believe,  however,  that  they 
follow  a  course  esseutially  similar  to  that  of  the  teeth. 

Fig.  58  (i). 


It  is  stated  by  Gegenbaur  that  in  Selachia  the  mucous 
membrane  of  the  mouth  is  clothed  with  spines  of  a  structure- 
similar  to  that  of  the  teeth,  and  that  these  spines  are  often 
limited  to  particular  regions,  extending  back  as  far  as  the 
pharynx — these  same  regions  in  Ganoids  and  Osseous  fishes 
being  occupied  by  conspicuous  teeth;  and  Hertwig  has. 
shown  that  the  dei-mal  spines  are  developed  in  a  manner 
precisely  analogous  to  that  described  in  the  teeth,  save  that 
the  germs  are  even  less  specialised. 

In  Teleostei  or  Osseous  Fishes. — In  passing  from 
the  consideration  of  the  development  of  the  tooth  germs  of 
Elasmobranch  to  those  of  Osseous  fishes,  the  first  difference 
to  be  noted  is  this  :  whereas  in  the  former  each  tooth  germ 
was,  so  far  as  the  enamel  germ  is  concerned,  derived  fi-om  that 
of  the  next  older  tooth,  in  the  latter  each  enamel  germ  often 
arises  independently  and,  as  it  were,  de  novo.     At  all  events, 

(')  Section  of  lower  jaw  of  young  Dog-fish,  showing  the  continuity  of 
the  dermal  sijines  of  the  skin  under  the  jaw,  with  the  teeth  which  lie 
above  and  over  its  end. 


THE    DEVELOPMENT    OF    THE    TEETH. 


121 


SO  far  as  my  own  investigations  go,  no  connection  has  been 
traced  between  the  germs  of  teeth  of  different  ages  ;  but 
Heiucke  says  that  in  the  Pike  new  enamel  organs  may  be 
derived  from  older  ones. 

This  independent  origin  of  an  indefinite  number  of  teeth, 
having  no  relation  to  their  predecessors,  is  only  certainly 
known  to  occur  in  the  osseous  fish  :  of  the  development  of 
the  teeth  of  Ganoid  fish  nothing  is  known. 

The  oral  epithelium,  which  varies  much  in  its  thickness 
and  in  other  characters  in  difierent  fishes,  sends  down  a 
process  which  goes  to  form  an  enamel  organ,  whilst  a  dentine 
papilla  in  rising  up  to  meet  it,  comes  to  be  invested  by  it  as 
■with  a  cap.  The  after-history  of  the  process  depends  much 
on  the  character  of  tooth  which  is  to  be  formed.  If  no 
enamel,  or  but  a  rudimentary  coat  of  enamel,  is  to  be  formed. 


Fig.  59  (\). 


the  cells  of  the  enamel  organ  remain  small  and  insignificant, 
as  in  the  mackerel.  If,  on  the  other  hand,  a  partial  invest- 
ment of  enamel  is  found  upon  the  perfected  tooth,  such 
for  instance,  as  the  little  enamel  tips  upon  the  teeth  of  the 
eel  (see  Fig.  90),  then  the  after-development  of  the  enamel 
organ  is  very  instructive. 

(')  Tooth-igerra  of  an  eel.  d.  Neck  of  enamel  organ,  e.  Enamel  cells. 
«.  Cap  of  enamel,  h.  Cap  of  dentine,  e.  Eudiraentary  enamel  cells 
opposite  to  that  part  of  tlie  dentine  germ  where  no  enamel  will  be  formed. 


122  A    MANUAL    OF  DENTAL    ANATOMY. 

Opposite  to  the  apex  of  the  dentine  papilla,  where  the 
enamel  cap  is  to  be,  the  cells  of  the  enamel  organ  attain  to 
a  very  considerable  size,  measuring  about  -^]y^  of  an  inch  in 
length  3  below  this  the  investing  caj)  of  enamel  organ  does 
not  cease,  but  it  is  continued  in  a  sort  of  rudimentary  con- 
dition. Thus,  although  the  enamel  organ  invests  the  whole 
length  of  the  dentine  papilla,  its  cells  only  attain  to  any 
considerable  size  opposite  to  the  point  where  the  enamel  is 
to  be  formed.  The  knowledge  of  this  fact  often  enables  an 
observer  to  say,  from  an  inspection  of  the  tooth  genu, 
whether  it  is  probable  that  the  perfected  tooth  will  be  coated 
with  enamel  or  not.  In  any  case  an  enamel  organ  will  be 
there,  but  if  no  euamel  is  to  be  formed,  the  individual  cells 
do  not  attain  to  any  considerable  degree  of  differentiation 
from  the  epithelium  elsewhere ;  in  other  words,  the  whole 
enamel  organ  will  partake  of  the  chai-acter  of  the  lower 
portion  of  that  represented  in  the  figure  of  the  tooth  germ 
of  the  eel. 

Although  of  course  there  are  many  differences  of  detail 
arising  from  the  very  various  situations  in  which  teeth  are 
developed  in  fish,  so  great  uniformity  pervades  all  which 
I  have  examined,  that  we  may  at  once  pass  on  to  the  con- 
sideration of  the  development  of  the  teeth  of  reptiles,  merety 
adding  that  it  is  not  altogether  true  to  say  that  the  teeth  of 
fish  in  their  development  exemplify  transitory  stages  in  the 
development  of  mammalian  teeth. 

In  Reptiles. — So  far  as  the  appearances  presented  by 
the  individual  germs  go,  there  are  few  difierences  worthy 
of  note  to  be  found  in  the  present  class  by  which  they  are 
distinguishable  from  those  of  either  fish  or  mammals.  The 
enamel  organ  is  derived  from  the  oral  epithelium,  the  den- 
tine organ  from  the  submucous  tissue  in  a  manner  very 
similai* ;  nevertheless,  there  ai'e  points  in  the  relation  which 
the  successional  tooth  germs  bear  to  one  another,  and  to  the 
teeth  already  in  situ,  which  are  of  some  little  interest.     The 


THE   DEVELOPMENT    OF    THE    TEETH.  123 

constant  succession  of  new  teeth  met  with  amongst  almost 
all  reptiles  renders  it  easy  to  obtain  sections  showing  the 
teeth  in  all  stages  of  growth  :  upon  the  inner  side  of  the  jaw- 
there  will  be  found  a  region  occupied  by  these  forming  teeth 
and  by  nothing  else,  which  may  be  called  "  area  of  tooth  deve- 
lopment ; "  this  is  bounded  on  the  one  side  by  the  bone  and 


Fig.  go  (i). 


A 


teeth  which  it  carries,  and  on  the  other  by  a  more  or 
less  sharply  defined  wall  of  fibrous  connective  tissue.  In 
the  newt,  for  example  (Fig.  60),  to  the  left  of  the  tooth  in 
use  are  seen  four  tooth  sacs,  in  serial  order,  the  youngest 
being  nearest  to  the  median  line  of  the  mouth.  As  the  sacs 
increase  in  size  they  appear  to  undergo  a  sort  of  migration 
towards  the  edge  of  the  jaw,  while  simultaneoiisly  new  ones 
are  constantly  being  developed  beyond  them.     In  the  newt, 

•« 
(^)  Section  of  upper  jaw  of  Triton  crlstatus  (newt).     To  the  inner  .side 
of  the  tooth  attached  to  the  bone  are  tliree  younger  tooth  germs. 


124  A    MANUAL    OF   DENTAL    ANATOMY. 

the  ^ingrowth  of  the  epithelium  is  obviously  the  first  step 
apparent;  this  ingrowth  of  a  process  of  epithelium  takes. 
place  in  close  relation  with  the  "neck"  of  an  older  enamel 
organ  (i.e.,  the  contracted  band  of  epithelium  which  remains, 
for  some  time  connecting  the  new  enamel  organ  with  the 
epithelium  whence  it  was  derived).  New  enamel  organs  are 
therefore  not  derived  directly  from  the  epithelium  of  the 
surface,  but  from  the  necks  of  the  enamel  organs  of  their 
predecessors. 

In  the  newt,  the  developing  teeth  spread  out  for  a  con- 
siderable distance  towards  the  palate,  and  thus,  being  free 
from  crowding,  the  relations  of  the  enamel  organs  of  three 
or  four  successional  teeth  of  serial  ages  may  be  studied  in  a 
single  section  ;  and  the  ai'rangement  so  disclosed  may  be 
advantageously  compared  with  that  seen  in  the  dog-fish  (see 
Fig.  57). 

The  tooth  sac  of  the  newt  is  a  good  example  of  the  sim- 
plest form  of  tooth  sac,  consisting  solely  of  an  enamel  organ 
and  a  dentine  germ,  without  any  especial  investment.  The 
*'  sac"  is  wholly  cellular,  and  on  pressure  breaks  up,  leaving 
nothing  but  cells  behind  it.  The  cells  of  the  enamel  organ 
are  large,  and  resemble  those  of  the  eel ;  the  teeth  of  newts. 
have  a  partial  enamel  tip,  like  those  of  the  fish  referred  to, 
but  differing  from  them  in  being  bifurcated,  as  is  very 
early  indicated  by  the  configuration  of  the  enamel  organ. 

In  the  frog  there  is  a  peculiarity  in  the  manner  in  which 
the  two  jaws  meet,  the  edentulous  lower  jaw,  which  has  ne 
lip,  passing  altogether  inside  the  upper  jaAv  and  its  svip- 
ported  teeth,  and  so  confining  the  area  of  tooth  develop- 
ment within  very  narrow  limits.  Consequently  I  have  been 
unable  to  satisfy  myself  whether  the  new  tooth  germs,  or 
rather  their  enamel  organs,  are  derived  from  those  of  their 
predecessoi's,  or  spring  up  de  novo — analogy  would  indicate 
the  former,  but  appearances  tend  towards  the  latter  sup- 
position. 


TEE   DEVELOVMENT    OF    THE    TEETH.  125 

In  the  lizards  the  new  tooth  germs  are  formed  a  very  long 
way  beneath  the  surface,  so  that  the  neck  of  the  enamel 
organ  becomes  enormously  elongated,  for  the  dentine  papilla 
is,  just  as  in  the  newt,  situated  at  first  quite  at  the  level  of 
the  floor  of  the  area  of  tooth  development.  The  teeth  of 
the  lizards  have  a  more  complete  investment  of  enamel, 
hence  the  enatnel  cells  are  developed  upon  the  side  of  the 
dentine  germ  to  a  much  lower  point  than  in  the  newt.    The 

Fig.  61  (•). 


germs  also  acquire  an  adventitious  capsule,  mainly  derived 
from   the   condensation   of    the   connective   tissue   around 


{})  Transverse  section  of  the  lower  jaw  of  common  English  Snake,  e. 
Inward  dipping  process  of  epithelium.  /.  Oral  epithelium.  1,  2,  3,  &c. 
Tooth  germs  pf  various  ages.  8.  Tooth  in  place,  cut  somewhat  obliquely, 
so  that  its  tip  aj^parently  falls  short  of  its  surface,  and  does  not  project 
above  the  mucous  membrane. 


126  A    MANUAL    OF   DENTAL    ANATOMY. 


them,  which  is  pushed  out  of  the  way  as  they  grow  larger. 
The  further  progress  of  the  tooth  germ  being  identical  with 
that  of  mammalia,  its  description  may  be  for  the  present 
deferred. 

In  ophidian  reptiles  (snakes)  several  peculiarities  are  met 
with  which  are  very  characteristic  of  the  order.  A  snake's 
method  of  swallowing  its  food  would  seem  to  render  the 
renewal  of  its  teeth  frequently  necessary;  although  I  do 
not  know  of  any  data  by  which  the  prolmble  durability  of 
an  individual  tooth  could  be  estimated,  the  large  number 
of  teeth  which  are  developing  in  reserve,  all  destined  to 
succeed  to  the  same  spot  upon  the  jaws,  would  indicate  that 
it  is  short. 

I  have  seen  as  many  as  seven  successional  teeth  in  a  single 
section,  and  their  arrangement,  particularly  in  the  lower 
jaw,  which  undergoes  great  displacement  while  food  is  being 
swallowed,  is  very  peculiar.  The  numerous  successional 
tooth  sacs,  instead  of  being  spread  out  side  by  side,  as  in 
the  newt,  are  placed  almost  vertically,  and  in  a  direction 
parallel  with  the  surface  of  the  jaw-bone ;  they  are,  more- 
over, contained  in  a  sort  of  general  investment  of  connective 
tissue ;  a  species  of  bag  to  keep  them  from  displacement 
during  the  expansion  of  the  mouth. 

The  inward  growing  process  of  oral  epithelium  enters  this 
case  of  tooth  sacs  at  its  top  ;  and  may  be  caught  sight  of 
here  and  there  as  its  prolongations  wind  their  way  by  the 
sides  of  the  tooth  sacs  to  the  bottom  of  the  area.  Here 
the  familiar  process  of  the  formation  of  an  enamel  organ  and 
dentine  papilla  may  be  observed,  in  no  essential  point  differ- 
ing from  that  which  is  to  be  seen  in  other  animals. 

That  the  derivation  of  each  enamel  organ  is  from  a  part  of 
that  of  its  predecessor  is  very  obvious ;  the  dentine  organs 
are  formed  in  relation  with  the  enamel  germs,  but  apparently 
independently  of  one  another. 

As  the  tooth  sacs  attain  considerable  dimensions,  a  curious 


THE   DEVELOPMENT    OF    THE    TEETH. 


127 


alteration  iu  position  takes  place  ;  instead  of  preserving  a 
vertical  position,  they  become  recumbent,  so  that  the  form- 
ing tooth  lies  moi-e  or  less  parallel  with  the  long  axis  of  the 
jaw.     The  utility  of  such  an  arrangement  is  obvious  :  were 


the  tooth  to  remain  erect  after  it  has  attained  to  some  little 
length,  its  point  would  probably  be  forced  through  the 
mucous  membrane  when  the  mouth  was  put  upon  the 
stretch  ;  but  while  it  lies  nearly  parallel  with  the  jaw  no 
such  accident  can  occur. 

The  tooth  does  not  resume  the  upright  position  until  it 
finally  moves  into  its  place  upon  the  summit  of  the  bone. 

Q)  Developing  teeth  of  a  Snake.  /.  Oral  epithelium,  c.  Neck  of  the 
enamel  organs,  b.  Dentine  pulp.  o.  Enamel  cells,  d.  Dentine.  1,  2. 
Very  young  germs.     3,  4.  Older  germs. 


128  A    MANUAL    OF   DENTAL    ANATOMY. 


As  has  already  been  mentioned,  there  is  a  well-developed 
enamel  organ  with  large  enamel  cells  :  from  these  a  thin 
layer  of  enamel  is  formed  and  thus  the  thin  exterior  layer 
upon  the  teeth  of  snakes  is  true  enamel,  and  not,  as  has 
been  usually  supposed,  cementum. 

Many  points  in  the  development  of  the  teeth  of  reptiles  I 
have  passed  over  very  briefly  for  the  want  of  space ;  a  more 
full  account  of  my  observations  will  be  found  in  the  Philo- 
sophical Transactions  for  1875. 

In  Mammalia  the  earliest  changes  which  will  ultimately 
result  in  the  formation  of  a  tooth  are  traceable  at  a  very 

Fig.  63  ('). 


early  period  ;  before  the  commencement  of  ossification,  the 
lower  jaw  consisting  solely  of  Meckel's  cartilage  imbedded  in 
embryonic  tissue,  and  the  lateral  processes  which  become 
the  upper  maxillary  bones  having  but  just  reached  as  far 
as  the  median  process  which  constitutes  the  intermaxillary 
Iwne.  That  is  to  say,  about  the  fortieth  or  forty-fifth  day 
(in  the  human  subject),  in  the  situation  corresponding  to 
the  future  alveolar  border,  there  appears  a  slight  rounded 
depression,  extending  the  whole  length  of  the  jaw,  it  and 

{})  Embryo  at  end  of  fifth  week  after  Carpenter.  1,  2.  First  two 
visceral  arches,  a.  Saperior  maxillary  process,  t.  Tougne.  b.  Eye. 
c.   Lateral  nasofrontal  process,     nf.  Nasofrontal  process. 


THE   DEVELOPMENT    OF    THE    TEETH.  129 

its  elevated  borders  being  formed  by  tin  increase  in  the 
thickness  of  the  layer  of  epithelial  cells  ;  while  in  trans- 
verse sections  the  proliferation  of  epithelial  cells  is  found  to 
have  been  even  more  energetic  in  a  direction  downwards 
into  the  substance  of  the  jaw  than  it  is  upwards,  so  that  a 
cul-de-sac  of  epithelium  dips  into  the  embryonic  sub-mucous 
tissue/ 

In  a  certain  sense,  then,  there  is  a  dental  groove,  hwi  it 
is  not  the  same  thing  as  that  described  as  such  in  the  text- 
books, and  it  therefore  better  to  abstain  from  applying  that 
or  any  other  name  to  the  shallow  fuiTow  of  which  we  are 
now  speaking,  which  is  almost  filled  up  with  spherical  or 
flattened  cells,  the  deepest  layer  being,  however,  columnar 
cells.  From  the  bottom,  or  the  side  near  the  bottom  of  the 
depression,  an  inflection  of  epithelial  cells  takes  its  origin, 
which  does  not  dip  downwards  vertically,  but  inclines  in- 
wards. This  secondary  narrow  inflection  of  epithelium, 
which  in  section  closely  resembles  a  tubular  gland,  consti- 
tutes the  rudiment  of  the  future  enamel  organ ;  a  prolife- 
ration of  the  cells  at  its  deepest  extremity  speedily  takes 
place,  so  that  it  expands,  attaining  somewhat  the  form  of  a 
Florence  flask  (Fig.  64).  It  should,  however,  be  noted,  that 
while  the  inflection  of  epithelium  takes  place  around  the 
entire  circumference  of  the  jaw,  so  that  that  which  appears 
in  sections  like  a  tubular  gland  is  really  a  continuous  sheet 
or  lamina  of  epithelium,  the  dilatations  of  its  extremity, 
which  I  have  compared  to  a  Florence  flask,  occur  only  at  the 
several  points  where  teeth  will  nltimately  be  developed. 

The  cells  upon  the  periphery  are  columnar,  polygonal 
cells  occupying  the  central  area  of  the  enlargement.     Very 

(')  The  epithelium  having  been  I'emoved  by  maceration  or  by  keeping  a 
specimen  in  dilute  spirit,  a  groove  would  result,  and  this  is  pi-obably  what 
was  seen  and  described  by  Goodsir  as  the  "primitive  dental  groove"  : 
but,  as  the  student  will  gather  from  the  text,  there  is  at  do  time  any  such 
thing  as  a  deep  open  groove  like  that  described  by  him,  unless  it  results 
from  maceration  and  consequent  partial  destruction  of  the  specimen. 


130 


A    MANUAL    OF   DENTAL    ANATOMY. 


soon  the  terminal   enlargement,  as  it  grows  more  deeply 

iiito  the  jaw,  alters  in  form ;   its  base  becomes  flattened, 

Fio.  64  (1). 

/  2 


"" ^#^^ 


l- 

c  - 


'm^T^^-^ 


J- 


A 


^^ 


and  the  borders  of  the  base  grow  down  more  rapidly  than 
the  centre,  so  that  its  deepest  portion  presents  a  concavity 
looking  downwards  ;  it  might  be  compared  to   a  bell,  sus- 

i})  Three  stages  in  the  development  of  a  mammalian  tooth  germ  (from 
Frey).  a.  Oral  epithelium  heaped  np  over  germ.  6.  Younger  epithelial 
cells,  c.  Deep  layer  of  cells,  or  rete  Malpighi.  d.  Inflection  of  epithelium 
for  enamel  germ.  t.  Stellate  reticulum.  /.  Dentine  germ.  g.  Inner 
portion  of  future  tocth  sac.  h.  Outer  portion  of  future  tooth  sac. 
i.  Vessels  cut  across.     A-.  Bone  of  jaw. 


THE   DEVELOPMENT    OF    THE    TEETH.  131 


pended  from  above  by  the  thin  cord  of  epithelium  -which 
still  coimects  it  with  the  epithelium  of  the  surface,  or  it 
might  in  section  be  described  as  crescentic,  the  horns  of  the 
crescent  being  long,  and  looking  downwards.  Coincident 
with  the  assumption  of  this  form  by  the  enamel  germ,  is 
the  appearance  of  the  dentine  germ  ;  but  it  will  facilitate 
the  description  of  the  process  to  pursue  a  little  farther 
the  development  of  the  enamel  organ. 

The  cells  on  its  periphery  remain  prismatic  or  columnai', 
but  those  in  its  centre  become  transformed  into  a  stellate 
network,  in  which  conspicuous  nuclei  occupy  the  centre  of 
ramified  cells,  the  processes  from  which  anastomose  freely 
with  those  of  neighbouring  cells  (see  Fig.  65).  This  conver- 
sion of  the  cells  into  a  stellate  reticulum  is  most  marked 
quite  in  the  centre  of  the  enamel  organ  ;  near  to  its  surfaces 
the  processes  of  the  cells  are  short  and  inconspicuous. 

The  transformation  of  the  cells  occupying  the  centre  and 
constituting  the  bulk  of  the  enamel  organ  into  a  stellate 
reticulum  goes  on  progressing  from  the  centre  outwards,  but 
it  stops  short  of  reaching  the  layer  of  columnar  cells  which 
constitute  the  surface  of  the  enamel  organ,  next  to  the  den- 
tine papilla;  a  narrow  layer  of  unaltered  cells  which  remain 
between  the  stellate  cells  and  the  columnar  enamel  cells 
constituting  the  "  stratum  intermedium." 

Thus  far  the  cells  constituting  the  periphery  of  the 
enamel  organ  are  alike  :  they  are  columnar  or  prismatic, 
but  from  the  time  of  the  appearance  of  the  dentine  papilla 
those  which  come  into  relation  with  it  become  much  more 
elongated  and  greatly  enlarged,  while  those  round  the  outer 
or  convex  surface  of  the  enamel  organ  do  not  enlarge ; 
indeed,  according  to  some  autliors,  they  even  commence  to 
atrophy  even  at  this  early  period.  The  cells  which  lie  like 
a  cap  over  the  dentine  germ  or  "papilla"  as  they  elongate 
and  their  nuclei  recede  to  their  extremities,  take  on  the 
character   to   be  presently  described  as   belonging   to  the 


132  A    MANUAL    OF  DENTAL    ANATOMY. 


"  enamel  cells,"  (enamel  epithelium,  internal  epithelium  of 
the  enamel  organ). 

The  enamel  organ,  then,  consists  (proceeding  from  with- 
out inwards)  of  an  "  external  epithelium,"  a  "  stellate  reti- 
culum," a  "  stratum  intermedium,"  and  an  "  internal  epithe- 
lium," the  external  and  internal  epithelia  being  continuous 
at  the  edges  or  base  of  the  enamel  organ,  while  at  its  summit 
the  external  e]Dithelium  remains  still,  through  the  medium 
of  the  "  neck  of  the  enamel  organ,"  in  continuity  with  the 
cells  of  the  "  stratum  Malpighi." 

Thus  the  enamel  organ  is  entirely  derived  from  the  oral 
epithelium,  with  which,  by  means  of  its  "neck,"  it  long 
retains  a  connection,  so  that  it,  and  whatever  products  it 
may  afterwards  give  rise  to,  are  obviously  to  be  regarded  as 
"  epithelial  structures."  But  it  is  the  enamel  organ  alone 
which  is  directly  derived  from  the  epithelium  ;  the  origin  of 
the  dentine  germ  is  quite  distinct. 

In  the  embryonic  tissue  of  the  jaws,  some  little  distance 
beneath  the  surface,  and  at  a  point  corresponding  to  that 
ingrowth  of  cells  and  subsequent  enlargement  of  the  same 
which  goes  to  form  the  enamel  organ,  appears  the  first  trace 
of  a  dentine  germ.(^)  This  appears  as  a  mere  increase  in 
the  opacity  of  the  part,  without  at  first  any  visible  structural 
change,  and  it  occupies  the  concavity  of  the  enamel  organ. 
Thus  the  dentine  germ  appears  early,  indeed  almost  simul- 
taneously with  the  formation  of  a  definite  enamel  organ,  but 
the  enamel  organ  is  far  in  advance  of  it  in  point  of  structural 
diiferentiation,  and  the  earliest  changes  which  result  in  the 
formation  of  the  enamel  organ  are  strikingly  visible  before  a 
dentine  germ  can  be  discovered.     According  to  Dursy  the 

Q)  The  term  "dental  papilla,"  although  eminently  convenient,  is  asso- 
ciated with  an  erroneous  feature  of  the  older  views  upon  tooth  develoiJ- 
ment ;  where  it  is  employed  in  the  following  pages,  the  student  must 
guard  against  the  misconception  that  free  papillte  at  any  time  exist  in  any 
animal. 


THE   DEVELOPMENT    OF    THE    TEETH.  133 


dark  halo  which  becomes  the  dentine  bulb  is,  like  the  inflec- 
tion of  epithelium  which  forms  the  enamel  germ,  continuous 
all  round  the  jaw,  while  eventually  it  develops  into  promi- 
nences at  the  points  corresponding  to  the  enamel  germs  of 
future  teeth,  and  atrophies  in  their  interspaces. 

From  the  base  of  the  dentine  bulb  prolongations  pass  out- 
ward aiid  slightly  upwards,  so  that  they  in  a  measure  embrace 
the  free  edge  of  the  enamel  organ,  and  at  a  somewhat  later 
period  they  grow  uj^wards  till  they  fairh-  embrace  the  whole 
enamel  organ, 

These  prolongations  of  the  base  of  the  dentine  bulb  are 
the  rudiments  of  the  dental  sac.  In  their  origin,  therefore, 
the  dental  sac  and  the  dentine  organ  ai-e  identical,  and 
spring  from  the  submucous  tissue  :  they  contrast  with  the 
enamel  organ,  which,  as  before  said,  is  derived  from  the  oral 
epithelium. 

To  recapitulate  briefly  the  facts  which  are  now  established 
beyond  all  question,  the  earlj^  mammalian  tooth  g-erm  con- 
sists of  three  parts,  one  of  wdiich,  the  enamel  organ,  is 
derived  from  the  epithelium  of  the  surface ;  the  other  two, 
the  dentine  organ  and  the  dental  sac,  originate  in  the  midst 
of  solid  embryonic  tissue  at  a  distance  from  the  surface. 

The  enamel  organ  is  formed  by  a  rapid  increase  of  cells  at 
the  bottom  of  a  process  which  dips  in  ft'om  the  stratum 
Malpighi  of  the  oral  epithelium ;  the  dentine  germ  and  the 
dental  sac  are  formed  in  close  contiguity  to  this  enamel 
organ  from  the  submucous  tissue. 

If  there  was  a  "  basement  membrane "  demonstrable  at 
this  early  period  (which  there  is  not)  the  enamel  organ  and 
the  dentine  organ  would  lie  upon  the  opposite  sides  of  it. 

The  description  of  the  appearance  of  the  several  parts  of 
the  tooth  germ  has  brought  us  to  the  period  at  which  cal- 
cification first  commences,  but  before  proceeding  further, 
it  will  be  well  to  examine  more  minutely  the  structure  of 
the  several  organs  in  which  calcification  takes  place. 


A    MANUAL    OF  DENTAL    ANATOMY 


ENAMEL    ORGAN. 

The  enamel  organ,  as  has  ah-eady  been  stated,  forms  a 
cap-like  investment  to  the  dentine  bulb,  and  it  is  itself 
thickest  over  the  apex  of  the  latter,  tliiuning  down  some- 
what as  it  approaches  the  base. 

It  is  entirely  surrounded  by  an  epithelial  layer,  which 
upon  the  inner  surface  applied  to  the  dentine  bulb  consists 
of  much  elongated  columnar  cells,  and  takes  the  name  of 
internal  epithelium  of  the  enamel  organ,  and  upon  its  outer 
surface  the  name  of  external  epithelium  of  the  enamel  organ. 
The  greater  bulk  of  the  enamel  organ  consists  of  a  stellate 
tissue,  which  passes  gradually  through  the  medium  of  a 
layer  of  rounded  cells,  the  stratum  intermedium,  into  the 
enamel  cells,  or  internal  ejnthelitim.  The  essential  portion 
of  the  enamel  organ  is  this  layer  of  "  enamel  cells,"  which 
by  their  calcification  give  rise  to  the  enamel,  and  in  lower 
animals,  such  as  most  if  not  all  reptiles,  the  whole  enamel 
organ  is  rei^resented  by  little  else  than  this  layer  of  "  enamel 
cells." 

The  cells  of  the  internal  epithelium  (enamel  cells)  form 
an  exceedingly  regular  and  perfect  columnar  epithelium, 
the  individual  cells  becoming  by  result  of  their  mutual 
apposition  very  symmetrical  hexagons. 

They  are  four  or  five  times  as  long  as  they  are  broad,  and 
the  nucleus,  which  is  large  and  oval,  occupies  that  end 
which  is  farthest  from  the  dentine.  It  is  said  by  Waldeyer 
that  the  sides  of  the  cells  only  are  invested  by  membrane, 
the  protoplasm  being  without  investment  at  its  two  ends. 

Towards  the  base  of  the  dentine  germ,  where  the  internal 
epithelium  merges  into  the  external  ej)ithelium,  the  cells  are 
not  so  much  elongated,  and  they  then  pass  gradually  into 
the  cubical  form  of  these  latter  cells.  At  their  attached 
extremities  the  enamel  cells  are  prolonged  into  processes 
which  are  continuous  with  the  cells  of  the  stratum  inter- 


THE   BEVELOPMEMT    OF    THE    TEETH.  135 


medium,  so  that  it  may  fairly  be  concluded  that  the  enamel 
cells,  as  they  are  used  up  in  the  formation  of  enamel,  are 
recruited  from  the  cells  of  this  layer. 

The  "stratum  intennedium"  consists  of  cells  intermediate 
in  character  between  those  of  the  bordering  epithelium  and 
the  stellate  reticulum ;  they  are  branched,  but  less  conspicu- 
ously so  than  the  stellate  cells,  with  which  on  the  one  hand 
they  are  continuous,  on  the  other  with  the  enamel  cells. 

The  stellate  cells  proper  are  characterised  by  the  great 
length  of  their  communicating  processes,  and  the  interspace 
of  the  meshes  is  occupied  by  a  fluid  rich  in  albumen,  so  that 
the  consistence  of  the  whole  is  little  more  than  that  of  jelly  ; 
as  the  structure  in  question  constitutes  the  major  part  in 
bulk  of  mammalian  enamel  organs,  these  have  been  called 
the  enamel  jellies,  or  enamel  pulps. 

Fig.  65  [^). 


The  function  and  destination  of  this  portion  of  the  enamel 
organ  is  not  veiy  clear :  enamel  can  be  very  Avell  formed 
without  it,  as  is  seen  amongst  reptiles  and  fish,  and  even 
in  mammalia  it  disappears  prior  to  the  completion  of  the 
enamel,  so  that  the  external  and  internal  epithelia  come  into 
contact.  It  has  been  supposed  to  have  uo  more  important 
function  than  to  fill  up  the  space  subsequently  taken  up  by 
the  growing  tooth.     (See  page  156). 

{})  Cells  of  the  stellate  reticulum  of  the  enamel  organ.  From  Frey's 
Histology. 


136  A    MANUAL    OF  DENTAL    ANATOMY 


The  external  epithelium  of  the  enamel  organ  is  com] 
of  cells  cubical  or  rounded  in  form,  and  is  of  little  interest 
save  in  that  it  is  a  matter  of  controversy  what  becomes  of 
it.  Waldeyer  holds  to  his  opinion  that,  after  the  disap- 
pearance of  the  enamel  pulp  and  the  stratum  intermedium, 
it  becomes  applied  to  the  enamel  cells,  and  on  the  comple- 
tion of  the  enamel  becomes  cornified  and  converted  into 
Nasmyth's  membrane.  Xolliker  and  Legros  and  Magitot 
dissent  from  this  opinion,  the  latter  stating  that  the  atrophy 
of  these  cells  commences  early,  and  that  they  actually  dis- 
appear \mov  to  the  complete  atrophy  of  the  organ.  For 
reasons  which  I  have  given  elsewhere,  I  do  not  agree  with 
Waldeyer  in  this  matter,  but  rather  with  Magitot.  The 
external  ej^ithelium  was  seen  by  Nasmyth,  Huxley,  and 
Guillot,  but  it  was  not  very  fully  described  until  investigated 
by  Robin  and  Magitot. 

So  simple  a  matter  as  the  vascularity  or  non-vascularity 
of  the  enamel  organ  is  not  yet  settled ;  Wedl  asserts  that  it 
contains  no  vessels,  Magitot  and  Legros  sharing  this  opinion ; 
Dr.  Lionel  Beale,  on  the  other  hand,  stating  that  a  vascular 
network  lies  in  the  stratum  intermedium. 

The  inner  surface  of  the  enamel  organ,  where  it  is  applied 
to  the  dentine  bulb,  presents  a  perfectly  smooth  outline,  but 
its  outer  surface  is  indented  by  numerous  papillary  projec- 
tions, into  which  enter  blood  vessels  of  the  dental  sacculus. 
These  papillae  are  homologous  with,  and  continuous  with 
those  of  the  gum  ;  they  may  sometimes  be  traced  along  the 
neck  of  the  enamel  germ,  and  it  is  believed  that  they 
exercise  an  important  influence  on  the  formation  of  the 
enamel,  to  which  I  shall  again  recur. 

The  narrow  attenuated  line  of  cells  by  which  the  enamel 
organ  retains  its  connection  with  the  stratum  Malpighi, 
whence  it  was  derived,  varies  much  in  length  and  direction 
in  different  animals ;  in  man  it  is  short  and  straight,  in 
the  calf  it  is  laroer,  and  undulates  in  its  course.     It  does 


THE    DEVELOPMENT    OF    THE    TEETH. 


137 


not  remain  quite  that  simple  line  of  cells  of  which  it  con- 
sisted when  first  formed,  but  varicosities,  made  up  of  poly- 
hedral cells,  bud  out  from  it. 

The  origin  of  the  dental  germs  of  the  permanent  teeth 
remains  to  be  described  :  the  twenty  teeth  which  have 
deciduous  predecessors  being  derived  from  parts  of  the 
germs  of  these,  the  twelve  true  molai's  having  a  distinct 
origin.  About  the  sixteenth  week  of  intra-uterine  life,  from 
the  neck  of  cells  which  connects  the  enamel  organ  of  the 
temporary  enamel  germ  with  the  stratum  Malpighi,  there 
buds  out  a  secondary  inflection  of  epithelium,  similar  in 
appearance  to  the  first   rudiment  of  the  enamel   germ   of 

Fig.  66  ('). 


the  milk  tooth ;  it  passes  down  to  the  inner  side  of  the 
temporary  tooth  sac,  and  by  undergoing  a  series  of  changes 
in  all  respects  analogous  with  those  resulting  in  the  forma- 
tion of  the  temporary  tooth  germ,  gives  rise  to  the  i:)erma- 
nent  tooth  germ, 

(')  Dental  germ  of  temporary  tooth  of  an  Armadillo,  showing  its  enamel 
organ,  and  the  enamel  germ  of  the  successional  permanent  tooth  to  the 
left  of  it. 


138  A    MANUAL    OF  DENTAL    ANATOMY. 


The  first  permanent  molar  germ,  howevei',  is  developed 
about  the  sixteenth  week  by  a  similar  budding  out  of  epi- 
thelium, from  that  same  primary  epithelial  lamina,  -whence 
the  temporary  enamel  germs  originated :  whilst  the  second 
pei*manent  molar  oinginates  from  the  neck  of  the  enamel 
organ  of  the  first  molar  after  a  long  interval,  i.e.,  about  the 
third  month  after  birth. 

The  enamel  germ  of  the  wisdom  tooth  is  similarly  derived 
from  the  neck  of  that  of  the  second  permanent  molar,  again 
after  a  long  interval ;  (about  the  third  year.     Magitot). 

The   accompanying  figure  represents   the   enamel  germ 

Fig.  6"  ('). 

c  a  J 


*   /  / 


for  a  permanent  tooth  budding  ofi"  from  the  neck  of  the 
enamel  organ  of  the  temporary  tooth.  Many  differences  of 
detail,  such  as  the  point  at  which  they  arise,  the  depth  to 

(1)  From  the  upper  jaw  of  a  kitten  about  the  time  of  birth,  a.  Oral 
epithelium,  i.  Bone  of  jaw.  c.  Neck  of  enamel  organ,  d.  Dentine 
papilla,  c.  Enamel  cells.  /.  Stellate  reticulum,  h.  Tooth  germ  of  the 
permanent  tooth,  the  enamel  organ  of  which  is  derived  from  the  neck  of 
that  of  its  predi 


THE   DEVELOPMENT    OF    THE    TEETH.  139 


which  they  penetrate  into  the  surrounding  parts,  and  other 
such  characteristics  exist  not  only  between  the  germs  of  teeth 
of  different  animals,  but  even  between  those  of  teeth  situated 
in  different  parts  of  the  mouth  of  the  same  animal,  so  that 
but  little  importance  is  to  l)e  attached  to  them. 


Dentine  Organ. 


The  dentine  germ,  or  dentine  bulb,  of  which  the  origin 
has  been  abeady  described,  at  first  was  nothing  more  than 

Fig.  63  ('). 


y 


a  part  of  the  submucous  tissue  of  the  jaw  which  had  become 
more  rich  in  vessels  and  cells  than  other  neighbouring  parts, 

(')  Tooth  sac  of  a  calf.  a.  Tooth  sac.  a^  a^.  Its  outer  and  middle 
portions,  b.  Stellate  cells  of  enamel  organ,  c.  External  epithelium  of 
enamel  organ,  d.  Internal  ei^ithelium  of  enamel  organ,  e.  Odontoblasts. 
/.  Dentine  bulb  in  papilla,  g.  Vessels  in  dentine  bulb.  t.  Points  where 
the  sac  becomes  fused  with  the  base  of  the  dentine  papilla. 


140  A    MANUAL    OF   DENTAL    ANATOMY. 

but  did  not  present  any  structures  essentially  different  from 
those  found  around  it.  It  very  speedily  assumes  the  form 
of  the  apex  of  the  future  tooth,  becoming,  if  it  be  a  canine, 
simjDly  conical,  if  a  tooth  with  two  cusps,  bicuspid;  and 
coincidently  with  these  changes  the  layer  of  cells  forming 
its  surface,  which  is  in  close  relation  with  the  enamel  cells, 
becomes  differentiated  from  the  parts  beneath  it. 

These  cells,  which  become  dentine  by  their  calcification, 
form  a  very  distinct  layer,  which,  after  the  commencement 
of  calcification,  adheres  more  strongly  to  the  formed  cap  of 
dentine  than  to  the  i-est  of  the  pulp,  and  so  is  often  pulled 
away  with  the  former  when  the  two  are  separated  ;  hence 
this  layer  of  cells  has  obtained  the  name  of  "  membrana 
eboris,"  or  membrane  of  the  ivory ;  but  the  student  must  • 
beware  of  falling  into  the  mistake  of  supposing  that  it  really 
is  a  "  membrane  "  properly  so  called. 

The  individual  cells,  which  collectively  constitute  the 
membrana  eboris,  are  called  odontoblasts ;  they  are  variable 
in  shape,  differing  even  in  the  same  animal  according  as 
the  formation  of  dentine  is  going  on  actively  or  not,  but 
are  always  much  longer  than  they  are  broad,  and  are  fur- 
nished with  oval  nuclei  near  to  their  deeper  ends.  The 
odontoblast  cells  are  furnished  with  several  processes,  named 
according  to  their  direction  ;  thus  the  process  which  pene- 
trates the  formed  dentine  is  called  the  ''dentinal  process," 
that  wliich,  at  its  opposite  pole,  passes  into  the  deeper 
part  of  the  pulp  where  it  communicates  with  other  cells, 
the  "  pulp  process,"  and  those  which  communicate  with 
neighbouring  odontoblasts,  "  lateral  processes."  Before 
entering  upon  a  detailed  description  of  the  transformation 
which  the  various  cells  undergo  in  their  conversion  into 
enamel,  dentine,  or  cementum,  it  will  not  be  o\it  of  place 
to  say  a  few  vrords  relative  to  the  process  of  calcification 
generally. 

But  before  doing  ro  it  may  perhaps  assist  the  student,  who  may 


THE   DEVELOPMENT    OF    THE    TEETH.  141 

be  perplexed  in  endeavouring  to  reconcile  the  statements  of  various 
authors,  to  give  a  succinct  history  of  the  views  from  time  to  time 
set  forth.  (') 

Before  the  time  of  Goodsir  (1838),  the  development  of  the  teeth 
was  described  by  Raschkow  somewhat  vaguely  as  proceeding  under- 
neath the  mucous  membrane,  he  did  not,  however,  trace  out  in 
what  precise  manner  the  several  parts  of  the  tooth  germ  originated. 
The  papers  of  Goodsii-  giving,  in  the  place  of  somewhat  vague  and 
general  notions,  a  very  definite  and  intelligible  description  of  ob- 
servations, was  accepted  without  question  by  most  anatomists,  if 
not  by  all.  Accordingly  we  find  in  all  the  text-books  at  and  after 
that  period,  and  ia  some  even  at  the  present  daj,  the  description 
given  by  Goodsir  reproduced  almost  without  alteration,  so  that  it 
wiU  be  worth  while  to  briefl'y  relate  what  his  views  were. 

He  believed  that  at  an  early  period  in  fcetal  life  there  appeared 
a  continuous  open  groove,  running  round  the  whole  circumference 
of  the  jaws  ;  that  from  the  bottom  of  this  groove  tliere  arose  iso- 
lated and  uncovered  papillae,  corresponding  in  number  to  the  milk 
teeth ;  that  these  papillaj  became  covered  in  by  the  deepening  of 
the  groove  and  the  meeting  of  its  two  edges  over  their  tops,  whilst 
at  the  same  time  transverse  septa  were  formed,  so  that  the  several 
papillae  became  enclosed  in  their  own  separate  follicles.  With  the 
details  of  the  process  as  described  by  him  we  are  not  concerned  ;  it 
will  suffice  to  remember  that  he  distmguished  the  four  stages  ;  a 
primitive  dental  groove,  a  papillary  stage,  a  follicular  stage,  and  an 
eruptive  stage  (the  latter  of  coiu-se  at  a  long  subsequent  period). 

Not  only  were  these  views  accepted  quite  without  question,  but 
they  were  even  extended  to  explain  the  development  of  the  teeth 
in  Reptiles  and  Fishes,  and  thus  in  the  OdontogTaphies  of  Professor 
Owen  and  Professor  Giebel  may  be  found  accounts  of  the  develop- 
ment of  the  teeth  in  reptiles  and  fish  which  are  perfectly  in  accoixl 
with  Goodsir's  theoiy,  but  which  in  fact  are  far  more  inaccurate 
than  the  same  theories  were  as  applied  to  mammalian  teeth. 

Even  so  careful  a  writer  as  Professor  Huxley,  who  was  the  fii-st 
to  point  out  that  these  stages  really  did  not  exist  either  in  the  frog, 
the  mackerel,  or  certain  other  fish,  accepted  them  without  question 
as  true  of  mammalia.  Marcusen  (^')  (1849)  gave  upon  the  whole  a 
correct  account  of  the  process,  referring  the  enamel  to  the  oral  epi- 
thelium, and  Professor  Huxley  (1854),  whilst  demonstrating  that 
the  stage  of  free  papillse  was  not  to  be  found  in  certain  fish  and 

(')  After  the  present  suminary  had  been  partly  prepared,  the  autbor 
met  with  the  veiy  excellent  resume  given  by  Messrs.  Legros  and  Magitot, 
from  which  he  has  received  much  assistance. 

{^)  In  the  rhaim  given  by  Messrs.  Legros  and  Magitot,  before  referred 
to,  due  credit  is  not  given  to  the  papers  of  Marcusen  and  Huxley  (1849, 
1854)  (although  they  are  alluded  to),  and  it  appears  to  roe  that  too  much 
is  given  to  that  of  Xatalis  (juillot  (1S58). 


142  A    MANUAL    OF  DENTAL    ANATOMY. 


reptiles  (a  fact  also  made  out  in  the  newt  by  Dr.  Beale),  clearly  and 
strongly  expressed  the  same  view  as  to  the  origin  of  the  enamel 
organ,  and  hence  of  the  enamel.  And  whilst  regretting  that  their 
hold  upon  the  minds  of  anatomists  has  been  so  strong  as  to  en- 
courage deductions  therefrom  going  wider  and  wider  of  the  mark,  I 
would  not  be  understood  to  set  small  value  upon  the  observations 
of  Arnold  and  Goodsir.  They  were  a  step  in  advance,  and  were 
probably  as  accurate  as  the  methods  of  investigation  then  in  use 
would  allow  of  :  moreover,  the  error  in  observation  is  very  easy  to 
account  for,  as,  the  epithelium  having  peeled  ofP  as  a  result  of  de- 
composition, or  the  use  of  weak  spirit,  the  state  of  things  left  does 
not  widely  differ  from  that  described  by  Goodsir. 

The  subject  rested  for  many  years  without  further  advances,  but 
in  18G3  Professor  Kolliker  demonstrated,  beyond  all  cavil,  the  real 
origin  of  the  enamel  organ  and  its  relations  to  the  oral  epithelium, 
the  dentine  organ,  and  the  dental  sac. 

His  views,  substantially  correct,  have  been  elaborated  by  Wal- 
deyer,  KoUmann,  Hertz,  Legros  and  Magitot,  Wedl,  and  others,  but 
only  in  minor  particulars  have  they  been  modified. 

The  development  of  the  teeth  of  reptiles  was  found  by  a  pupil  of 
M.  KoUiker's,  M.  Santi  Sirena,  to  have  several  features  in  accord 
with  that  of  mammalian  teeth  ;  my  own  researches  on  the  teeth  of 
Batrachia  and  Fish  and  Reptiles,  elsewhere  detailed,  have  proved  a 
striking  general  similarity  in  the  process  throughout  the  vertebrate 
kingdom,  though  they  are  not  in  accord  with  the  views  of  Pro- 
fessors Owen  and  Giebel. 

Dental  Follicle. — Tu  the  foregoing  accouut  little  mention 
has  been  made  of  the  tooth  follicle  or  tissue  forming  a 
capsule-like  investment  around  the  dentine  germ  and  enamel 
organ.  At  an  early  period  of  development  the  tissue  form- 
ing the  dentine  papilla  of  a  mammalian  tooth  is  seen  to  be 
prolonged  outwards  and  upwards  from  its  base  (see  h  in 
Fig.  64) ;  these  processes  appear  to  grow  rapidly  upwards, 
so  as  to  embrace  the  enamel  organ;  but  whether  this  is 
really  so,  or  whether  it  is  merely  that  the  ill-defined  tissue 
in  which  the  dentine  forming  organ  has  itself  originated  is 
in  this  region  also  becoming  more  pronounced,  it  is  hard  to 
say.  This  up-growth  from  the  base  of  the  dentine  papilla 
is  the  first  appearance  of  a  special  dental  sacculus,  which  is 
thus  derived  from  a  source  identical  with  that  of  the  forma- 
tive organ  of  the  dentine. 


Fio.  fi9  (1) 


rni.ied  from  Wil.Uxd   (IMil.  ^  /Mt- 

tiithful  to  mturt  ,  it  is  sotcii  dia^'i  mil 

magnifying  power  einplnyed  is  iiitrmlii 

:.  DentiiiP.  germ,  ^vitli   its  iHnilfi-  df 

4.  Points  where  flie  inii'i'  ipitli"; 

continuous.     :>.  Kimmel  rells  (.r  i 

organ.      7.  Htellate  retieuliim  <>( 

organ.    9.  Connective  tissue  aroun 

gum  (Of/);    this  eonsti 

jaw.    11.  Bone  of  low 


J  \w  \N!)  Ui  \i  1  oi  iM  1>MK  fooiH  Of  \  Lamb, 
t.  lUt.  Jled.  IbUs).  In  its  outline'^  tlie  figure  ib 
that  more  ot  structuic  than  could  be  seen  with  the 


■d  deiil 


liat  is  c-iil.M  (lii 

1-J.     IVno>trlll,l 

theyouiig  tooth.    14.  External  skin  with  its 
of  mouth. 


ormed  enamel, 
ii.niiel  become 
iiiii  of  enamel 
ito  the  enamel 
ontiiiuuu.s  above  with  that  of  the 
ic.    10.  Vessels  and  nen-es  of  the 


iiiiii.     ii.   K 
N.   Tapillai-y  projei 


.inw, 


13.  Heap  of  epithelium  over 
JIusciilar  bundles  from  floor 

[Tijfiw'p.  14;;, 


THE   DEVELOPMENT    OF    THE   TEETH.  143 

While  these  changes  are  going  on,  the  tooth  sac  is  becoming 
lodged  in  a  widely  open  gutter  of  bone,  which  is  being  rapidly 
formed  at  its  sides  and  under  its  base.  If  at  this  stage 
(see  Fig.  69)  the  gum  be  stripped  off  the  jaws,  the 
developing  tooth  capsules  are  torn  off  with  it,  from 
which  they  ai'e  inseparable  except  by  actual  cutting,  thus 
leaving  the  gutter  of  bone  quite  bare  and  empty.  In  fact 
the  capsule  or  sac  which  encloses  the  tooth  germ  consists  of 
almost  the  whole  of  the  connective  tissue  which  intervenes 
between  the  special  dentine  and  enamel  germs  and  the  bone. 

In  the  first  instance  the  follicle  wall  is  only  distinguished 
from  the  connective  tissue  external  to  it  by  being  somewhat 
richer  in  cells,  vessels,  and  fibrillar  elements ;  being,  in  foct, 
more  condensed  or  more  compact.  The  sacs,  when  at  their 
fullest  development,  are  divisible  into  two  layers,  an  outer 
thin  firm  wall,  and  an  inner  looser  tissue,  not  very  dense.  At 
the  base  of  the  tooth  sac,  the  follicle  wall  is  not  separable 
nor  distinguishable  from  the  base  of  the  dentine  papilla 
with  which  it  blends.  The  follicle  wall  is  richly  vascular  ; 
and  over  the  surface  of  the  enamel  organ  it  is  prolonged 
inwards  in  the  form  of  villous  or  papilliform  eminences  (8 
in  Fig.  69),  projecting  into  the  external  epithelium  of  the 
enamel  organ  ;  to  these  prominences,  which  are  analogous  to 
the  papillae  on  the  free  surface  of  the  gum,  some  authors 
attach  much  importance,  as  having  an  influence  upon  the 
direction  of  the  enamel  prisms,  and  so  regulating  the 
pattern  formed ;  but  this  view  is  by  no  means  universally 
accepted.  The  internal  or  softer  and  looser  portion  of  the 
follicle  wall,  which  has  a  consistency  but  little  firmer  than 
that  of  the  stellate  reticulum  of  the  enamel  organ,  is  much 
developed  in  Ruminants,  where  there  is  to  be  a  deposition 
of  coronal  cement.  This  differentiation  of  a  portion  of  the 
dental  sac  is  thought  by  Messrs.  Legros,  Robin,  and  Magitot 
to  be  sufficiently  pronounced  to  justify  its  designation  as  a 
distinct  "  cement  organ." 


A    MANUAL    OF   DENTAL    ANATOMY. 


The  Cement  Organ. 

Cementum  is,  according  to  tliese  authors,  developed,  just 
as  bone  is,  in  two  distinct  methods. 

Where  it  is  not  to  be  very  thick,  and  is  to  clothe  roots, 
the  ossification  takes  place  in  membrane  (the  alveolo 
dentar  periosteum),  but  where  it  is  to  form  a  thick  layer 


Fig.  70  f'l 


over  the  crown,  as  in  lluuiinants,  a  cartilaginous  cement 
organ  is  formed,  and  we  have  a  calcification  analogous  to 
formation  of  bone  in  cartilage. 

Thus  the  cement  organ  is  found  in  those  animals  only 
which  have  coronal  cement,  such  as  the  Herbivora.  In  a 
calf  embryo  about  the  time  that  dentine  calcification  is 
commenced,  there  may  be  distinguished  beneath  the  follicle 
wall  and  above  the  enamel  organ  a  greyish  layer  of  tissue, 
thick  enough  to  be  distinguishable  with  the  naked  eye,  and 
of  firmer  consistence  than  the  enamel  organ,  from  which  it 
also  differs  in  being  richly  vascular. 

{})  Cement  organ  of  a  calf  (aftei-  Magitot),  a.  Fibroid  matrix. 
h.  Cartilage  cells  and  cap.si;]<  s. 


THE   DEVELOPMENT    OF    THE    TEETH.  145 

But  though  it  exists  at  this  early  period,  it  is  not  till 
later,  wheu,  after  the  completion  of  the  dentine  and  enamel 
immediately  beneath  it,  its  own  function  is  about  to  come 
into  play,  that  it  attains  to  its  characteristic  structnre. 
This  M,  Magitot  designates  as  fibro-cai-tilaginous,  as  there 
appear  in  it  characteristic  cartilage  cells  or  chondroplasts, 
containing  one,  two,  or  rarely  three  cells,  which  have 
spherical  or  ovoid  nuclei. 

In  those  creatures  which  have  cementum  upon  the  roots  of 
the  teeth  only,  no  special  cement  organ  exists,  but  osteoblasts 
which  calcify  into  cementum  are  furnished  by  the  tooth  sac. 

It  is  said  that  the  inner  layer  of  the  tooth  sac  is  concerned 
in  the  formation  of  the  cement ;  that  the  outer  layer,  con- 
jointly with  the  surrounding  connective  tissue,  is  converted 
into  the  alveolo-dentar  periosteum,  but  I  cannot  myself 
recognise  'the  justice  of  this  distinction  in  practice.  In 
human  teeth  the  parts  of  the  follicle  wall  or  sac  cease  to  be 
distinctly  distinguishable  at  a  comparatively  early  period, 
and  their  importance  is  not  such  as  to  call  for  very  detailed 
description. 

Another  structure,  once  thought  important,  and  now 
known  to  be  a  mere  bundle  of  dense  fibrous  tissue,  is  the 
" guhernaculum."  The  permanent  tooth  sacs,  during  their 
growth,  have  become  invested  by  a  bony  shell,  which  is 
complete,  save  at  a  point  near  their  apices,  where  there  is  a 
foramen.  Through  this  foramen  passes  a  thin  fibrous  cord, 
very  conspicuous  w4ien  the  suiTounding  bone  is  broken 
away,  which  is  called  the  "  guhernaculum,"  from  the  notions 
entertained  by  the  older  anatomists  that  it  was  concerned 
in  directing  or  effecting  the  eniption  of  the  tooth.  The 
gubemacula  of  the  front  permanent  tooth  sacs  perforate 
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A    MANUAL    OF  DENTAL    ANATOMY. 


Calcification. 

A  tissue  is  said  to  be  "  calcified  "  when  the  organic  stnic- 
tures  of  which  it  is  composed  are  hardened  and  stiffened  by- 
impregnation  with  salts  of  lime.  The  impregnation  with 
lime  salt  may  go  on  so  far  that  the  residual  organic  matrix 
is  reduced  to  a  very  small  proportion,  as  is  exemplified  in 
the  case  of  adult  enamel,  in  which  the  organic  constituents 
make  up  only  from  one  to  three  per  cent,  of  the  whole,  so 
that  practically  the  enamel  wholly  disappears  under  the  in- 
fluence of  an  acid  j  or  the  organic  matrix  may  persist  in 
fiufl&cient  quantity  to  retain  its  structural  characteristics 
after  the  removal  by  solution  in  an  acid  of  its  salts,  as  is 
the  case  with  dentine,  bone,  and  cementum.  There  are 
two  ways  in  which  a  calcified  structure  may  be  built  up  : 
the  one  by  the  deposition  of  the  salts  in  the  very  substance 
of  a  formative  organ,  which  thus  become  actually  converted 
into  the  calcified  structure ;  the  other  by  a  formative  organ 
shedding  out  from  its  surface  both  the  organic  and  inorganic 
constituents,  and  thus,  so  to  speak,  excreting  the  resultant 
tissue. 

An  example  of  the  latter  method  is  to  be  found  in  the 
shells  of  many  mollusks,  in  which  the  mantle  secretes  the 
shell,  and  is  able  to  repair  fractures  in  it,  without  itself 
undergoing  any  apparent  alteration  ;  while  the  formation  of 
dentine,  bone,  and  enamel  (')  are  examples  of  calcification 
by  conversion. 

The  insoluble  salts  of  lime  are  altered  in  their  behaviour 
by  association  with  organic  compounds,  a  fact  which  was 
first  pointed  out  by  Rainie,  and  has  been  more  recently 
worked  out  by  Professor  Harting  and  Dr.  Ord. 

If  a  solution  of  a  soluble  salt  of  lime  be  slowly  mixed 
with  another  solution  capable  of  precipitating  the  lime,  the 

(1)  All  observers  are  not,  however,  agreed  as  to  the  formation  of  the 
enamel.     (Cf.  page  157.) 


THE  DEVELOPMENT    OF    THE    TEETH. 


resultaut  lime  salt  will  go  down  as  au  amorphous  powder, 
or,  imder  some  circumstances,  in  minute  crystals.  But  in 
the  presence  of  gelatine,  albumen,  and  many  other  organic 
compounds,  the  form  and  physical  character  of  the  lime 
salts  are  materially  altered,  and  in  the  place  of  an  amor- 
phous powder  there  are  found  various  curious  but  definite 
forms,  quite  unlike  the  character  of  crystals  produced  with- 
out the  intervention  of  the  organic  substance. 

Mr.  Rainie  found  that  if  calcium  carbonate  be  slowly 
formed  in  a  thick  solution  of  mucilage  or  albumen  the  re- 
sultant salt  is  in  the  form  of  globules,  laminated  in  structure, 
so  that  the  globules  may  be  likened  to  tiny  onions ;  these 
globules,  when  in  contact,  becoming  agglomerated  into  a 
single  laminated  mass,  it  appearing  as  if  the  laminae  in 
immediate  apposition  blended  with  one  another.  Globular 
masses,  at  one  time  of  mulberry-like  form,  lose  the  in- 
dividuality of  their  constituent  smaller  globules,  and  become 
smoothed  down  into  a  single  mass  ;  and  Mr.  Rainie  suggests 
as  an  explanation  of  the  laminated  stnictm-e  that  the  smaller 
masses  have  accumulated  in  concentric  layers  which  have 
subsequently  coalesced ;  and  in  the  substitution  of  the 
globular  for  the  amorphous  or  crystalline  form  in  the  salt  of 
lime  when  in  contact  with  various  organic  substances, 
Mr.  Rainie  claimed  to  find  the  clue  for  the  explanation  of 
the  development  of  shells,  teeth,  and  bone.  At  this  point 
Professor  Harting  took  up  the  investigation,  and  found  that 
other  salts  of  lime  would  behave  in  a  similar  manner,  and 
that  by  modifying  the  condition  of  the  experiment  very 
various  forms  (i)  might  be  produced.  But  the  most  im- 
portant addition  to  our  knowdedge  made  by  Professor 
Harting  lay  in  the  very  peculiar  constitution  of  the  "  calco- 
spherites,"  by  which  name  he  designated  the  globular  forms 
seen  and  described  by  Rainie.     That  these  are  built  up  of 

(M  Thus  lie  was  successful  in  artificially  producing  "dumb-bell"' 
crystals. 


A    MANUAL    OF  DENTAL    ANATOMY. 


concentric  laminae  like  an  onion  has  already  been  mentioned, 
and  Mr.  Rainie  was  aware  that  albumen  actually  entered 
into  the  composition  of  the  globule,  since  it  retained  its 
form  even  after  the  application  of  acid. 

But  Professor  Harting  has  shown  that  the  albumen  left 
behind  after  the  treatment  of  a  calcospherite  with  acid  is 
no  longer  ordinary  albumen ;  it  is  profoundly  modified,  and 
has  become  exceedingly  resistant  to  the  action  of  acids, 
alkalies,  and  boiling  water,  and  in  fact  resembles  chitine, 
the  substance  of  which  the  hard  skins  of  insects  consist, 
rather  than  any  other  body. 

For  this  modified  albumen  he  proposes  the  name  of 
"  calcoglobulin,"  as  it  appears  that  the  lime  is  held  in  some 
sort  of  chemical  combination,  for  the  last  traces  of  lime  are 
retained  very  obstinately  when  calcoglobulin  is  submitted 
to  the  action  of  acids. 

The  "calcospherite,"  then,  has  a  true  matrix  of  calco- 
globulin, which  is  capable  of  retaining  its  form  and  structure 
after  the  removal  of  the  great  bulk  of  the  lime. 

Now  it  is  a  very  suggestive  fact  that  in  the  investigation 
of  calcification  we  constantly  meet  with  structures  remark- 
able for  their  indestructibility  :  for  example,  if  we  destroy 
the  dentine  by  the  action  of  very  strong  acids,  or  by 
variously  contrived  processes  of  decalcification,  puti*efaction, 
&c.,  there  remains  behind  a  tangled  mass  of  tubes,  the 
"  dentinal  sheaths "  of  Neumann,  which  are  really  the 
immediate  walls  of  the  dentinal  tubes. 

Or  if  bone  be  disintegrated  by  certain  methods  there 
remain  behind  large  tubes,  found  to  be  the  linings  of  the 
haversian  canals  (Kolliker),  and  small  rounded  bodies, 
recognisable  as  isolated  lacunse ;  and  in  the  cuticula  dentis 
we  have  another  excellent  example  of  this  peculiarly  inde- 
structible tissue. 

In  point  of  fact,  as  will  be  better  seen  after  the  develop- 
ment of  the  dental  tissue  has  been  more  fully  described,  on 


TH-E   DEVELOPMENT    OF    THE    TEETH.  151 

the  borderland  of  calcification,  between  the  completed  fully 
calcified  tissue  and  the  formative  matrix  as  yet  unimpreg- 
nated  with  lime,  there  very  constantly  exists  a  stratum  of 
tissue  which  in  its  physical  and  chemical  properties  very 
much  resembles  "  calcoglobulin." 

It  should  also  be  noted  that  globulai*,  spherical  forms  are 
very  constantly  to  be  seen  at  the  edges  of  the  thin  cap  of 
forming  dentine,  and  may  be  also  traced  in  and  around  the 
interglobular  spaces  (see  Fig.  34) ;  moreover,  isolated 
spherules  of  lime  salt  have  been  described  by  Messrs.  Robin 
and  Magitot  as  occun-ing  abundantly  in  the  young  pulps  of 
human  teeth,  as  well  as  those  in  the  herbivora,  where  their 
presence  was  noted  by  Henle. 


Calcification  of  the  Examel, 

Although  the  calcification  of  the  dentine  commences 
before  that  of  the  enamel,  it  will  be  convenient  to  describe 
that  of  the  enamel  first,  as  being  a  somewhat  simpler  and 
more  easily  intelligible  process. 

As  has  already  been  mentioned,  I  am  distinctly  of  opinion 
that  the  enamel  is  formed  by  the  actual  conversion  of  the 
cells  of  the  enamel  organ  into  enamel,  but  as  this  view  is 
not  held  by  all  who  have  written  upon  the  subject,  I  will 
first  mention  the  alternative  theory,  namely,  that  the  enamel 
is  in  some  sense  secreted  or  shed  out  by  these  cells.  In 
support  of  this  latter  theory  the  names  of  no  less  authorities 
than  Professor  Huxley,  KoUiker,  Wenzel,  and  Magitot,  may 
be  adduced,  but  the  grounds  on  which  their  decisions  are 
based  are  appearances  susceptible  of  a  different  interpreta- 
tion. Kolliker  considers  that  the  cells  do  not  undergo  any 
direct  conversion,  but  that  the  enamel  is  shed  out  from  the 
ends  of  the  enamel  cells,  the  enamel  fibres  therefore  corre- 


152  A    MANUAL    OF  DENTAL    ANATOMY. 

spouding  in  size  and  being  continuous  with  tiie  enamel  cells 
whence  they  were  shed  out. 

Professor  Huxley's  reason  for  doubting  the  direct  con- 
version of  the  enamel  cells  into  enamel  was  that  a  membrane 
could  be  raised  from  the  surface  of  growing  enamel,  at  any 
period  of  its  development,  by  the  use  of  acid  reagents,  this 
membrane  necessarily  intervening  between  the  formed 
enamel  and  the  enamel  cells  ;  hence  he  denied  that  the 
enamel  organ  contributed  in  any  way  directly,  though  it 
might  indirectly,  to  the  development  of  the  enamel 

To  the  nature  of  this  "membrane  "  I  shall  have  again  to 
refer,  so  that  for  the  present  it  will  suffice  to  say  that  the 
structure  in  question  cannot  be  demonstrated,  and  in  fact 
has  probably  no  existence,  prior  to  the  use  of  the  reagent. 

The  cells  of  the  internal  epithelium  of  the  enamel  organ 
or  enamel  cells  have  been  already  in  some  measure  de- 
scribed :  they  are  elongated  cells,  forming  a  very  regular 
columnar  epithelium,  and  are  hence  rendered  hexagonal  by 
mutual  apposition ;  they  vary  in  their  length  and  diameter 
in  different  animals. 

To  secure  uniformity  of  nomenclature,  the  name  adaman- 
toblast  has  recently  been  proposed  for  them,  as  being  better 
comparable  with  the  term  odontoblast  and  osteoblast. 

Although  they  are  connected  with  the  cells  of  the  stratum 
intermedium  by  a  process  at  their  base,  they  often  adhere 
more  strongly  to  the  enamel,  when  once  this  has  begun  to 
be  formed,  than  to  the  rest  of  the  enamel  organ,  so  that 
when  a  dental  sac  is  opened  the  enamel  cells  are  most  easily 
obtained  by  scraping  the  surface  of  the  enamel.  The  cells 
thus  torn  away  often  have  tapering  processes  at  the  ends 
directed  towards  the  enamel,  which  were  first  described  by 
my  father,  and  go  by  the  name  of  "  Tomes'  processes."  The 
cells  are  also  slightly  enlarged  at  these  extremities,  especially 
if  they  have  been  immersed  in  glycerine  or  any  such  fluid 
which  causes  their  shrinkage,  for  this  end  of  the  cell  having 


THE   DEVELOPMENT    OF    THE    TEETH. 


153 


received  a  partial  impregnation  with  lime  salt  at  its  peri- 
phery, and  so  being  rigid,  is  unable  to  contract  with  the  rest 
of  the  cell.     These  enlarged,  everted  ends,  often  show  a  very 

Fig.  71  (}). 


sharp  contour,  their  trumpet-like  mouths  tending  to  confirm 
the  statement  of  Waldeyer  that  the  protoplasm  of  the  cell 
is  not  covered  in  by  membrane  at  its  ends.  The  impregna- 
tion with  calcareous  salts  commences  at  the  free  end  of  the 

Fw.  72  (-). 


enamel  cell,  and  at  the  periphery  before  the  central  portion, 
and  it  is  to  this  fact  that  the  existence  of  "  Tomes'  pro- 
cesses "  is  due,  for  when  the  enamel  cell  is  dragged  away 
from  the  foi-med  enamel  prism,  it  separates  across  the  line 
of  calcification  ;  and  thus  the  axial  part  of  the  cell,  when 
torn  away,  projects  out  further  than  its  peripheiy,  in  con- 

(')  Enamel  cells  with  Tomes'  processes. 

(^)  Enamel  cells  ;  the  two  on  the  right  have  been  shrunk  by  immersion 
in  glycerine,  and  present  the  open  tmmpet-shaped  ends  described  in  the 
text. 


154  A    MANUAL    OF  DENTAL    ANATOMY. 

sequence  of  calcification  having  extended  less  far  at  this 
central  portion  of  the  cell. 

In  other  words,  if  the  forming  enamel  were  freed  from 
the  adherent  enamel  cells,  its  surface  would  be  pitted, 
each  little  pit  marking  the  centre  of  an  enamel  prism  ;  and 
if  a  thin  section  of  this  immediate  surface  could  be  taken 
off,  it  would  be  pierced  with  holes  at  regular  intervals. 

The  enamel  cell  with  its  process  is  like  an  odontoblast  with 
a  very  short  dentinal  fibril,  which  has  been  pulled  out  of  the 
formed  dentine,  and  the  nature  of  the  "  Tomes'  processes  "  is 
well  illustrated  in  the  enamel  organs  of  marsupials.  It 
will  be  remembered  that  their  enamel  is  permeated  by  a 
large  number  of  canals,  which  become  continuous  at  the 
junction  of  the  dentine  and  enamel,  with  the  dentinal  tubes. 
Accordingly  the  enamel  cell  of  a  marsupial,  engaged  in  the 
formation  of  a  permanently  tubular  enamel,  is  just  like  an 
odontoblast  in  that  it  has  a  long,  fine  process,  pulled  out  of 
the  already  formed  enamel. 

As  the  youngest  part  of  the  enamel  has  by  no  means 
attained  to  its  full  hardness,  it  is  quite  possible  to  obtain,  in 
small  pieces,  sections  parallel  to  its  surface ;  the  nearer  they 
are  to  the  surface,  the  larger  will  be  the  perforations,  show- 
ing what  has  already  been  stated  respecting  calcification 
commencing  at  the  periphery  of  each  cell  to  be  true.  And 
it  is  possible,  by  the  use  of  an  acid,  to  obtain  such  sections 
upon  a  larger  scale,  for  under  the  influence  of  such  a  reagent, 
this  youngest  layer  of  the  enamel  peels  off  in  a  sheet, 
bringing  with  it  in  places  enamel  cells,  in  places  enamel 
prisms,  adhering  to  its  opposite  sides.  When  destitute  of 
adherent  enamel  cells  or  prisms,  this  so-called  membrane  is 
foraminated ;  and  the  processes  of  the  ends  of  the  enamel 
cells  are  fitted  into  and  passed  through  these  perforations. 

The  real  nature  of  the  membrane  which  could  be  raised 
from  the  surface  of  growing  enamel  was  first  demonstrated 
by  my  father,  and  his  explanation  has  been  accepted  by 


THE   DEVELOPMENT    OF    THE    TEETH.  155 

Waldeyer  and  other  authorities  ;  it  will  be  seen  that  this 
sheet,  produced  solely  by  the  destructive  action  of  reagents, 
corresponds  with  the  membrana  preformativa  of  some  writers 
(see  page  171),  and  with  the  membrane  described  by  Pro- 
fessor Huxley  as  intervening  between  the  enamel  cells  and 
the  enamel.  Hence  it  will  be  seen  that  the  fact  of  acids 
raising  a  membrane  from  the  surface  of  the  enamel  does  not 
really  militate  against  the  theory  that  the  enamel  is  due  to 
the  direct  convei'sion  of  the  enamel  organ  into  enamel. 

The  ends  of  the  enamel  cells  near  to  the  formed  enamel 
are  granular,  this  granularity  being  due  to  the  deposition  of 
particles  of  lime  salts,  as  is  indicated  by  its  clearing  up 
when  treated  with  acid. 

The  cells  on  the  one  side  of  the  membrane  readily  separate 
from  one  another,  adhering,  however,  slightly  by  their 
dilated  ends  (vide  supra),  and  the  fact  that  we  are  able  to 
isolate  the  youngest  layer  of  enamel  as  a  thin  sheet  is 
probably  to  be  explained  by  its  chemical  nature.  It  ap- 
pears to  belong  to  that  class  of  peculiarly  resistant  sub- 
stances which  are  to  be  found  on  the  borders  of  calcification, 
and  behaves  very  much  like  Professor  Hartiug's  "  calco- 
globulin"  (see  page  150);  at  all  events  it  may  safely  be 
said  to  have  undergone  some  chemical  change  preparatory 
to  the  i-eception  of  its  full  amount  of  lime  salts. 

The  calcification  of  the  enamel  should  be  so  complete 
that  itrf  fibrous  structure  is  but  slightly  apparent  in  longi- 
tudinal sections,  and  the  individual  fibres  should  appear 
structureless,  with  the  exception  of  the  feebly  marked 
striation  (see  page  50).  In  enamel  of  imperfect  structural 
character  the  centre  of  the  fibre  is  not  completely  calcified, 
the  arrest  of  development  having  taken  place  short  of  its 
full  conversion. 

The  stellate  tissue  of  the  enamel  organ  disappeai-s  some 
time  before  the  whole  thickness  of  the  enamel  is  formed, 
and  changes  go  on  in  the  latter  up  to  the  time  of  the  erup- 


156 


A    MANUAL    OF  DENTAL    ANATOMY. 


tion  of  the  tooth ;  the  enamel  of  a  tooth  prior  to  its  erup- 
tion having  a  chalky,  opaque  surface. 

The  enamel  of  the  teeth  of  reptiles  is  developed  from  an 
enamel  organ  which  at  no  time  possesses  any  stellate  tissue  ; 
this  is  also  the  case  in  all  fish  which  I  have  hitherto  ex- 
amined.    In  the  poison  fangs  of  snakes  the  enamel  cells, 


over  the  interior  of  the  poison  tube,  appeared  to  be  trans- 
formed into  a  stellate  reticukim,  which  change  in  this  case 
would  appear  to  be  a  retrograde  metamorphosis. 

The  nuclei  of  the  enamel  cells,  which  lie  at  the  estre- 
mities  furthest  from  the  enamel,  appear  to  recede  as  calcifi- 
cation goes  on ;  they  do  not  exercise  any  special  influence 
on  the  process  as  far  as  can  be  seen. 

(')  Transverse  section  of  the  tooth  sac  of  a  poison  fang  (Viper).  The 
crescentic  pulp  (a)  is  surrounded  by  a  layer  of  dentine  (cZ)  ;  external  to 
this  is  a  layer  of  columnar  enamel  cells,  which,  upon  the  exterior  of  the 
tooth,  upon  which  a  thin  layer  of  enamel  is  to  be  formed,  are  large  con- 
spisuous  cells.  Where  they  pass  in  between  the  horns  of  the  crescent, 
into  that  part  which  will  ultimately  be  the  poison  canal,  their  character 
is  lost,  and  their  place  taken  by  stellate  cells  (/).  No  enamel  is  formed 
in  this  latter  position. 


THE   DEVELOPMENT    OF   THE    TEETH.  157 


As  has  been  already  mentioned,  Kolliker  dissents  from  the  above 
account  of  the  calcification  of  the  enamel,  partly  on  the  gi-ound 
that  enamel  cells  may  be  seen  of  the  same  size  and  form  at  all 
stages  of  the  foiTuation  of  enamel. 

The  process  he  regards  as  one  of  secretion,  the  enamel  being  shed 
out.  so  to  speak,  from  the  free  end  of  each  enamel  cell ;  hence  the 
prisms  of  the  enamel  will  con-espond  in  size  and  number  with  the 
cells  of  the  enamel  epithelium  ;  the  processes  of  the  enamel  cells 
he  regards  as  being  fragments  of  this  hardened  secretion  which  are 
still  clinging  to  the  parent  cell. 

M.  Magitot  (Journal  de  Tanatomie  de  M.  Ch.  Robin,  1879)  has 
reyived  this  view,  describing  each  cell  as  tenninated,  towards  the 
forming  enamel,  by  a  little  plate  of  dense  material  through  which 
by  some  process  of  exosmosis  the  constituents  of  enamel  travel  out. 
He  notes  that  these  plates  often  cohere  so  as  to  form  a  sheet  (cf. 
page  IJJi),  but  says  nothing  of  their  being  perforated.  No  one, 
however,  who  had  seen  the  enamel  cell  of  a  marsupial  with  the 
tapering  process  five  or  six  times  as  long  as  itself  which  had  been 
pulled  out  of  the  young  enamel  would  be  satisfied  with  the  excre- 
tion theory. 

The  reasons  for  adopting  the  opposite  view  will  have  been  ga- 
thered from  the  text  ;  they  are,  in  brief,  the  occurrence  of  the 
•'  Tomes'  processes,"  especially  in  marsupials  ;  the  rigidity  of  the 
open  mouths  of  the  enamel  cells  ;  the  pitted  surface  of  the  youngest 
layer  of  enamel,  the  foraminated  membrane  which  can  be  raised 
from  it,  and  the  relation  of  these  facts  to  the  occurrence  of  the 
processes  of  the  enamel  cells. 

Schwann  believed  that  the  enamel  cell  was  constantly  increasing 
at  its  free  end  (/.<".,  that  next  to  the  enamel),  and  that  the  new 
growth,  or  youngest  part  of  the  cell,  is  calcified  as  fast  as  it  is 
foi-med  ;  this  view  differs  little  fi-om  that  of  Kolliker,  who  prefers 
to  express  it  by  saying  that  this  end  of  the  cell  is  constantly  shed- 
ding off  or  secreting  a  material  which  becomes  external  to  itself. 
My  father,  Waldeyer,  Hertz,  and  many  others,  believe  that  the  cell 
growth  takes  place  not  at  this  free  end,  but  at  the  attached  nucle- 
ated end,  and  that  it  is  the  oldest  portion  of  the  ceU  itself  which 
receives  an  impregnation  with  salts  and  forms  the  enamel. 

Professor  Huxley's  opinion  (page  152)  is,  I  take  it,  based  on  the 
fact  that  a  membrane  could  be  raised  from  the  surface  of  young 
enamel,  which  must  have  intervened  between  the  enamel  cells  and 
the  enamel  prisms  ;  if  my  father's  explanation  of  the  nature  of  this 
membrane  be  accepted,  the  difficulty  vanishes. 

My  o^vn  researches  upon  the  development  of  the  teeth  of  fishes 
also  furnish  evidence  tending  in  the  same  direction  :  as  has  been  al- 
ready mentioned,  the  enamel  cells  in  some  parts  of  the  enamel  org-ans 
of  certain  fish,  such  as  the  eel  and  perch,  and  certain  Batrachia, 
e.g.,  the  newt,  have  dimensions  very  greatly  exceeding  those  of  the 
cells  in  the  remainder  of  the  organ.     These  highly  developed  cells, 


158  A    MANUAL    OF   DENTAL    ANATOMY. 


three  times  as  long'  as  the  corresponding  cells  lower  down  upon  the 
dentine  papilla,  are  in  the  position  of  the  terminal  cap  of  enamel 
which  characterises  these  teeth.  Moreover  in  the  tooth  sac  of  the 
poison  fang  of  a  viper,  the  distribution  of  the  large  cells  coincides 
with  that  of  the  enamel  on  the  finished  tooth. 

Calcification  of  the  Dentine. — The  dentine  is  formed 
upon  the  aurface  of  the  dentine  bulb,  or  papilla,  from  with- 
out inwards,  so  that  no  portion  of  dentine  once  calcified  can 
receive  any  increase  in  external  dimensions ;  all  additions 
must  take  place  upon  the  interior  of  the  dentine  cap.  The 
nature  of  the  dentine  bulb  has  already  been  to  some  extent 
described ;  it  remains  to  consider  somewhat  more  minutely 
the  nature  of  its  surface.  The  cells  constituting  the  mem- 
brana  eboris,  to  which  Waldeyer  has  given  the  convenient 
name  of  "  odontoblasts,"  form  an  exceedingly  sharply  defined 
layer  upon  the  surface  of  the  dentine  wall,  being  aiTanged 
in  a  single  row ;  the  cells  immediately  beneath  them  differ 
strongly  from  them,  so  that  there  is  not  so  marked  an 
appearance  of  transitional  structure  as  may  be  seen  in  the 
stratum  intermedium  of  the  enamel  organ.  Nothing  what- 
ever like  the  linear  succession  of  formative  cells,  which,  by 
coalescence  at  their  ends  went  to  form  the  dentinal  tubes, 
as  described  l)y  the  older  waiters,  is  to  be  seen. 

The  odontoblast  cells  vary  in  form  according  as  the  den- 
tine formation  is  actively  going  on  or  not,  but  at  the  period 
of  their  greatest  activity  they  are  broad  at  the  end  dii-ected 
towards  the  dentine  cap,  so  as  to  look  almost  abruptly 
truncated.  The  several  processes  of  the  cells  have  already 
been  described ;  there  are,  however,  sometimes  several 
"  dentinal  processes "  proceeding  from  a  single  cell,  and 
Boll  has  counted  no  less  than  six. 

The  cells  are  finely  granular,  and  are,  according  to 
Waldeyer  and  Boll,  destitute  of  all  membrane  ;  the  nucleus 
is  oval,  lies  in  that  extremity  of  the  cell  which  is  farthest 
from  the  dentine,  and  is  sometimes  prolonged  towards  the 
dentinal  process  so  as  to  be  ovoid  or  almost  pointed. 


THE   DEVELOPMENT    OF    THE    TEETH.  159 


The  dentinal  process  passes  into  the  tubes  of  the  dentine, 
and  it  frequently  happens  that  when  the  membrana  eboris 
is  only  slightly  separated  from  the  dentine  these  processes, 

Fig.  74  (i). 


which  constitute  the  dentinal  fibrils,  may  be  seen  stretching 
across  the  interval  in  great  numbers. 

The  odontoblasts,  as  may  be  seen  from  figm-es  30  and  31, 
are  fitted  closely  together,  and  there  is  no  room  for  any  other 
tissue  between  them,  so  long  as  the  formation  of  dentine  is 
actively  going  on.  Prior  to  its  commencement,  however, 
the  cells  are  not  so  square  at  their  ends,  and  the  appear- 
ance of  the  thin  edge  of  such  a  pulp  suggests  the  idea  that 
they  are  bedded  in  a  transparent  and  structureless  jelly, 
which  projects  a  little  beyond  them.  To  render  my  mean- 
ing more  clear  by  a  homely  illustration,  the  surface  of  the 
pulp  at  this  stage  reminds  one  of  the  clear  jellies  put  upon 
the  table  with  strawberries  or  the  like  buried  in  them,  near 
to,  but  beneath,  the  surface.  But  no  such  substance  can 
be  seen  when  once  calcification  has  actively  set  in. 

When  the  pulp  has  completed,  for  the  time  being  at  all 
events,  the  formation  of  the  dentine,  the  odontoblast  cells 
become  more  elongated  and  more  rounded  in  their  outline 
and  taper  off  towards  and  into  the  dentinal  process,  instead 
of  having  truncated  ends. 

The  cells  figured  by  Lent  as  the  formative  colls  of  dentine 
I  regard  as  odontoblasts  taken  from  an  adult  tooth,  the 

(')  Isolated  odontoblast  cell. 


160  A    MANUAL    OF  DENTAL    ANATOMY. 

period  of  formative  activity  being  past,  and  I  am  inclined  to 
think  that  his  views  on  the  subject  of  development  are  open 
to  criticism,  as  being  based  upon  the  appearances  presented 
by  such  old  cells. 

Fig.  75  Q). 


The  dentine  is,  I  believe,  formed  by  the  direct  conversion 
of  the  odontoblast  cells,  just  as  is  the  enamel  by  that  of 
the  enamel  cells,  and  is  derived  from  them,  and  from  them 
alone. 

According  to  this  view,  which  is  supported  by  Waldeyer, 
Frey,  Boll,  Dr.  Lionel  Beale,  and  many  other  writers,  the 
dentinal  fibrils,  the  dentinal  sheaths,  and  the  matrix  between 
these  latter,  are  alike  derived  from  the  metamoi-phosis  of 
the  odontoblast  cell.  In  other  words,  the  three  structures 
in  question  may  be  taken  as  being  three  stages  in  the  con- 
version of  one  and  the  same  substance :  thus  we  have  the 
dentinal  fibril  in  its  soft  condition,  little  more  than  the 
unaltered  protoplasm  of  the  cell,  then  the  dentinal  sheath, 
one  of  those  peculiarly  resistant  substances  which  lie  on  the 
borders  of  calcification ;  and  lastly,  the  matrix,  the  com- 
pleted, wholly  calcified  tissue. 

That  some  such  relation  exists  seems  to  be  indicated  by 
the  fact  that  dentinal  tubes  once  formed  are  capable   of 

(')  Odontoblasts  in  situ.     After  'Wakleyer. 


THE   DEVELOPMENT    OF    THE    TEETH.  161 


further  calcification,  by  -^-hich  their  calibre  becomes  sensibly 
diminished.  Thus,  my  father  states  (speaking  of  the  incisor 
teeth  of  rodents),  "  the  tubes  which  proceed  from  the  pulp 
cavity  near  the  base  of  the  tooth,  are,  in  most  cases,  per- 
ceptibly larger  than  those  that  are  situated  higher  up  ; 
hence  it  follows  that,  as  the  latter  were  once  near  the  base 
of  the  tooth,  the  dentinal  tubes  undergo  a  diminution  of 
calibre  after  their  original  formation.  In  the  teeth  of  the 
Scim-idse  I  have  found  a  difference  of  size  amounting  to  a 
third  or  half  between  the  tubes  near  the  base  and  those 
near  the  surface  in  wear." 

And  Dr.  Lionel  Beale  calls  attention  to  the  fact  that  the 
hollows  of  the  canals  are  largest  nearest  to  the  pulp,  and 
smallest  at  the  periphery  of  the  tooth,  in  other  words,  at 
the  oldest  part ;  also  that  calcification  is  still  slowly  going 
on  even  in  advanced  life,  so  as  often  to  lead  to  the  oblite- 
ration of  the  peripheral  tubes.  There  is,  too,  the  statement 
of  Robin  and  Magitot,  that  the  teeth  become  more  rich  in 
calcareous  salts  as  age  advances,  so  that  analyses  of  human 
teeth  show  great  discrepancies. 

It  is  difiicu^lt  to  see  how  a  dentinal  tube  once  formed  can 
become  contracted  to  a  third  or  half  of  its  diameter  unless. 
we  believe  that  that  which  was  at  first  the  soft  tissue  (den- 
tinal fibril)  occupying  its  canal  may  become  at  its  periphery 
metamorphosed  into  "  dentinal  sheath,"  while  that  which 
was  ci-iginally  this  latter  has  passed  into  the  condition  of 
matrix.  Other  illustrations  of  this  fact,  observed  by  inde- 
pendent writei-s,  suggest  themselves  to  me ;  the  converti- 
bility of  the  dentinal  fibril  into  dentinal  sheath  and  of  the 
latter  into  matrix,  seems  to  be  of  necessity  implied  by  the 
narrowing  of  the  calibre  of  a  tube  once  formed,  for  the  tubes 
thus  naiTowed  present  no  special  character ;  their  walls  do 
not  appear  any  thicker,  nor  do  they  in  any  way  become  dif- 
ferent save  in  the  one  matter  of  diameter.  The  phenomena 
of  dental   caries  also  appear  to  lend  some  support   to  this. 


162  A    MANUAL    OF    DENTAL    ANATOMY. 


view,  that  dentinal  fibril,  dentinal  sheath,  and  matrix,  are 
but  three  ages  of  the  same  tissue. 

For  under  the  influence  of  caries  the  walls  of  the  tubes, 
invisible,  or  almost  so,  in  perfectly  healthy  dentine,  become 
apparent. 

As  I  have  elsewhere  expressed  it,  the  most  external 
portions  of  the  odontoblasts  undergo  a  metamorphosis  into 
a  gelatigenous  matrix,  which  is  the  seat  of  calcification, 
while  their  most  central  portions  remain  soft  and  unaltered 
as  the  fibrils.  Intermediate  between  the  central  perma- 
nently soft  fibril  and  the  general  calcified  matrix,  is  that 
jDortion  which  immediately  surrounds  the  fibi-il,  namely,  the 
dentinal  sheath;  as  expressed  by  Dr.  Lionel  Beale  they  are 
protoplasm,  formed  material,  and  calcified  formed  material. 

That  the  whole  of  the  dentine  is  derived  from  a  conversion  of  the 
•odontoljlast  cells  is  not  agreed  to  by  all  writers.  Thus  Kolliker  and 
Lent  believe  that  while  the  canals  and  their  contents  are  continua- 
tions of  the  odontoblasts,  the  matrix  is  a  secretion  either  from  these 
cells  or  from  the  rest  of  the  pulp,  and  so  is  an  "  intercellular  "  sub- 
stance. Their  view  is  therefore  intermediate  between  the  excretion 
und  conversion  theories  ;  and  KoUiker  goes  on  to  say,  "  since  the 
dentinal  cells  are  immediately  drawn  out  at  their  outer  ends  into 
the  dentinal  fibres,  and  do  not.  as  was  formerly  thought,  grow  out 
in  such  a  manner  that  the  dentinal  fibre  is  to  be  regarded  only  as 
the  inner  part  of  the  cell,  so  it  is  not  possible  to  derive  the  dentine 
immediately  from  the  cells."'  But  is  not  Professor  KoUiker  think- 
ing and  writing  of  those  aged,  spent  cells  which  his  jnipil  Lent 
figured  ?  No  one  could  speak  of  a  young,  active  odontoblast  as 
"  dra'rni  out  into  the  dentinal  fibril."  A  good  section  of  young 
developing  dentine  shows  that  the  cells  are  square  and  abrupt 
towards  the  dentine  :  they  do  not  taper  into  the  deirtinal  process  in 
the  smallest  degi-ee,  and  there  is  no  room  for  any  intercellular  sub- 
stance whatever. 

Hertz  coincides  with  KoUiker  in  regarding  the  matrix  as  a  "  se- 
cretion from  aU  the  dentinal  cells  in  common  which  stands  in  no 
definite  histological  relation  to  the  individual  cells,"  but  his  figui-e 
also  I  believe  to  be  representative  of  an  adult  inactive  surface  of 
pulp,  in  which  dentine  foiination  lip.d  almost  ceased. 

KoUiker  and  Lent  are  of  opinion  that  a  single  cell  is  suflJcient  to 
form  the  whole  length  of  a  dentinal  fibril,  not  having  seen  evidence 
of  active  cell  gTowth  in  the  siibjacent  layer  of  the  i)ulp.  from  which 
they  would  infer  that  the  membrana  eboris  was  supplemented  by 


THE   DEVELOPMENT    OF    THE    TEETH.  163 


new  cells  from  below.  In  the  latest  edition,  however,  KoUiker 
speaks  with  much  more  hesitation  on  this  point. 

Magitot  now  (1881)  holds  that  the  whole  of  the  dentine  is '•  a 
product  elaborated  by  the  odontoblasts,"  but  neither  secreted  by  nor 
formed  by  the  conversion  of  the  odontolilasts,  and  he  denies  the 
existence  of  the  sheaths  of  Xeumann  in  iotn. 

Klein  believes  that  the  odontoblast  forms  the  matrix  only, 
whilst  the  dentinal  fibrils  are  processes  continued  up  between  the 
odontoblasts  from  a  subjacent  layer  of  stellate  cells. 

Robin  and  Magitot  formerly  held  that  the  dentine  matrix  was 
fonned  by  the  transfoi-mation  of  the  odontoblast  cells,  but  that  the 
tubes  were  intcrspucfs  between  these  latter,  not  corresponding  with 
the  axes  of  the  cells. 

The  thinnest  layer  of  dentine,  such  as  may  be  found  at 
the  edges  of  the  dentine  cap,  is  soft  and  elastic,  and  so 
transparent  as  to  appear  structureless.  Where  it  has  at- 
tained a  somewhat  greater  thickness,  globules  begin  to 
appear  in  it,  which  are  small  in  the  thinner,  and  larger  in 
the  thicker  portion  of  the  dentine  cap.  As  they  are  actually 
in  the  substance  of  the  cap,  their  growth  and  coalescence 
obviously  go  on  without  any  very  immediate  relation  to 
the  cells  of  the  pulp ;  in  point  of  fact,  a  process  strictl}^ 
analogous  to  that  demonstrated  by  Mr.  Rainie  and  Professor 
Harting  (see  page  148),  is  going  on.  Thus,  in  the  formation 
of  the  first  skin  of  dentine,  a  stage  of  metamorphosis  pre- 
paratory to  impregnation  with  calcareous  salts  distinctly 
precedes  that  full  impregnation,  which  is  marked  by  the 
occurrence  of  globules  and  their  subsequent  coalescence. 
The  occurrence  of  these  globular  forms  and  consequent  large 
interglobular  spaces,  in  the  deeper  parts  of  adult  dentine,  is 
therefore  an  evidence  of  arrest  of  development  rather  than 
of  any  otherwise  abnormal  condition. 

When  the  formation  of  the  dentine  and  enamel  has  gone 
on  to  the  extent  of  the  crown  of  the  tooth  having  attained 
its  full  length,  the  reproduction  of  new  formative  pulp  (in 
teeth  of  limited  growth)  takes  place  only  over  a  contracted 
area,  so  that  a  neck,  and  finally  one  or  more  roots  are  the 
result,  of  its  conversion  into  tooth  substance.  In  teeth  of 
constant  growth,  however,  no  such  narrowiug  of  tlie  forma- 

M  2 


164  A    MANUAL    OF   DENTAL    ANATOMY. 

tive  pulp  takes  place,  but  the  additions  to  the  base  of  the 
tooth  are  of  constatit,  or  ever-increasing  dimensions,  as  is 
the  case  in  some  tusks,  -which  arc  of  conical  form. 

It  is  said  that  tlie  number  of  roots  which  would  have  been 
developed  at  the  base  of  a  particular  dentine  organ  may  be 
inferred  from  the  vessels,  i.  e.,  that  in  a  single  rooted  tooth 
the  vessels  would,  even  at  an  early  period,  form  a  single 
fasciculus,  in  a  double  rooted  one  similarly  they  would  be 
arranged  in  two  bundles,  so  that  the  ultimate  formative 
activity  will  be  exercised  around  one,  two,  or  three  centres 
of  nutrition.  I  am  not  however  able,  from  my  own  ob- 
servations, to  thrown  any  light  upon  this  matter. 


THE    CALCIFICATION    OF    VASO-DENTINE. 

During  the  conversion  of  the  membrana  eboris  into 
ordinary  hai'd  un vascular  dentine  the  vessels  of  the  formative 
pulp  recede,  so  that,  whilst  at  all  stages  a  cajiillary  plexus 
is  to  be  found  just  below  the  odontoblast  layer,  no  vessels 
are  to  be  found  amongst  the  cells  which  constitute  it. 

Nevertheless  a  moment's  reflection  will  show  that  (except 
in  the  eai-liest  stages,  before  any  dentine  is  formed)  the 
plexus  must  at  a  prior  time  have  occupied  the  place  now 
taken  possession  of  l"»y  the  inward  marching  odontol)lasts  and 
dentine. 

But  in  the  calcification  of  a  formative  pulp  into  vaso- 
dentine  this  recession  of  its  vessels  before  the  advancing 
border  of  calcification  does  not  take  place  ;  the  whole  vascular 
network  of  the  papilla  remains  and  continues  to  caiTy  blood 
circulating  through  it,  even  after  calcification  has  crept  up 
to  and  around  it. 

So  that  if  we  imagine  a  vascular  papilla  to  have  its 
stroma  suddenly  petrified  whilst  its  circulation  went  on  all 
the  same,  we  should  have  something  like  a  vaso-dentine. 

Just  as  in  hard  dentine,  the  odontoblast  layer  is  distinctly 


TEE   DEVELOPMENT    OF    THE   TEETH.  165 


marked  off  from,  the  rest  of  the  dentine  oi'gan,  and  the 
dentine  is  wholly  derived  from  its  conversion  into  calcified 
material,  so  that  the  difference  between  vaso-dentine  and 
hard  dentine  is  not  one  of  a  very  fundamental  character. 

Indeed,  as  we  have  seen  (p.  85),  the  same  formative  pulp, 
the  same  odontoblast  layer,  is  able  at  one  time  to  form  hard 
tubular  dentine,  at  another  vaso-dentine.  All,  therefore, 
that  has  been  before  said  of  the  calcification  of  odontoblasts 
will  apply  equally  to  those  of  a  vaso-dentine  pulp,  save  only 
that  in  a  typical  tissue  of  this  latter  kind  each  cell  calcifies 
solidly,  and  does  not  leave  the  axial  portion  soft,  to  remain 
as  a  dentinal  fibril. 

Of  the  development  of  Plicidentine  nothing  more  need 
be  said,  as  it  presents  no  peculiarities  which  are  not  the 
obvious  result  of  the  folding  of  the  surface  of  its  formative 
pulp-  

THE    CALCIFICATION    OF    OSTEO  DENTINE. 

With  the  exception  of  the  thin  external  layers  (see  fig.  47), 
which  are  develo])ed  from  a  superficial  layer  of  not  very 
highly  specialised  cells,  osteo-dentine  is  built  up  in  a 
manner  fundamentally  different  from  that  in  which  hard 
dentine,  plicidentine,  and  vaso-dentine,  are  constructed. 

For  it  is  not,  like  these,  a  surface  formation  ;  it  is  not 
laid  down  in  a  regular  manner  upon  the  exterior  of  a  pulp, 
and  it  has  no  relation  to  an  odontoblast  layer,  if  we  except, 
perhaps,  its  thin  exterior  shell. 

So  soon  as  this  has  been  formed  its  inner  surface  becomes 
roughened  by  trabecules  shooting  inwards  into  the  substance 
of  the  pulp,  which  speedily  becomes  traversed  completely 
by  them,  as  Avell  as  by  the  connective  tissue  bundles  which 
are  continuous  with  them.  Thus  the  pulp  being  pierced 
through  in  every  direction  by  these  ingrowths  cannot  be 
withdrawn,  like  the  pulp  of  a  hard  or  of  a  vaso-dentine  tooth, 


A    MANUAL    OF   DENTAL    ANATOMY. 


from  the  interior  of  the  dentine  cap.  Osteoblasts  clothe, 
like  an  epithelium,  the  trabeculfe  and  the  connective  tissue 
fibres  attached  to  them,  and  by  the  calcification  of  these  the 
osteodentine  is  formed. 

The  process  is  exactly  like  the  calcification  of  any  mem- 
brane bone,  and  the  connective  tissue  bundles  remind  one 
of  those  which  are  believed  to  be  the  occasion  of  the 
formation  of  Sharpey's  fibres  in  bone.  In  the  case  of  teeth 
which  are  going  to  be  anchylosed  to  the  subjacent  bone, 
these  fibres  run  continuously  from  the  interior  of  the  dentine 
cap  down  to  the  bone,  and  calcification  in  and  around  them 
binds  the  two  inseparably  together. 

It  is  interesting  to  note,  especially  in  connection  with 
the  fact  that  some  observers  believe  Sharpy's  fibres  to  be 
elastic,  that  the  hinged  teeth  of  the  pike  (see  fig.  88)  owe 
their  power  of  resilience  entirely  to  the  elasticity  of  these 
connective  tissue  bundles,  which  do  not  become  completely 
calcified,  although  at  an  early  stage  it  would  be  quite  im- 
possible to  say  whether  the  tooth  imder  observation  was 
going  to  be  anchylosed,  or  to  be  a  hinged  tooth  tied  down  by 
elastic  strings. 

The  Calcification  of  Cementum. — Just  as  is  the  case 
with  ])ones  elsewhere  in  the  body,  cenientum  may  be  formed 
in  two  distinct  ways,  by  membranous  ossification,  and  by 
ossification  in  a  fibro-cartilage,  the  former  method  obtaining 
upon  the  roots  of  teeth,  and  the  latter  upon  those  crowns 
where  the  cement  organ  described  by  Magitot  exists. 

At  the  time  when  the  crown  of  a  tooth  appears  through 
the  gum,  it  alone  is  complete,  and  the  root  has  yet  to  be 
calcified  ;  as  each  portion  of  dentine  of  the  root  is  completed 
it  is  coated  with  a  closely  adherent  vascvdar  membrane  which 
is  in  fact  the  follicle  wall,  and  Avhich  is  to  become,  when  the 
cement  is  formed,  the  alveolo-dentar  periosteum. 

The  inner  or  dentinal  face  of  this  membrane  presents  a 
layer  of  large  cells,  the  osteoblasts  of  Gegenbaur,  and  it  i& 


THE   DEVELOPMENT    OF    THE    TEETH.  167 


by  their  calcification  that  bone  or  cenjentum  is  directly 
formed.  These  osteoblasts  are  themselves  a  special  de- 
velopment where  bone  is  about  to  be  manufactured,  as  is 
cleaiiy  explained  in  the  following  extract  from  a  paper  by 
my  father  and  the  late  Mr.  De  Morgan,  who  termed  them 
osteal  cells : — 

"  Here  (towards  the  bone)  in  the  place  of  cells  with 
elongated  processes,  or  cells  arranged  in  fibre-like  lines,  we 
find  cells  aggregated  into  a  mass,  and  so  closely  packed  as  to 
leave  little  room  for  intermediate  tissue.  The  cells  appear 
to  have  increased  in  size  at  the  cost  of  the  processes  which 
existed  at  an  earlier  stage,  and  formed  a  bond  of  union  be- 
tween them.  Everywhere  about  growing  hone  a  careful  ex- 
amination will  reveal  cells  attached  to  its  surface,  tvhile  the 
surface  of  the  hone  itself  will  jyresent  a  series  of  similar  bodies 
ossified.  To  these  we  propose  to  give  the  name  of  osteal 
cells,  as  distinguished  from  lacunal  and  other  cells." 

Externally  to  the  osteoblast  layer,  but  still  very  near  to 
the  perfect  cementum,  lies  a  reticulum  or  network  made  up 
by  the  inosculating  branches  of  cells.  The  cells  have  largish 
round  nuclei,  and  are  each  furnished  with  three  or  four  homo- 
geneous processes,  so  that  the  tissue,  save  in  very  thin  sec- 
tions, looks  hopelessly  confused  from  the  interlacing  of  the 
cell  processes.  Many  of  these  processes  pass  into,  and  are  loit 
in  the  clear,  structureless  matrix  of  the  already  formed 
cementum ;  the  functions  which  they  perform  in  its  deve- 
lopment are  not  very  apparent,  as  they  do  not  correspond 
to  anything  which  can  be  traced  in  the  completed  tissue. 

Externally  to  the  fine-meshed  net-work  which  has  been 
Avell  figured  and  described  by  Dr.  Lionel  Beale,  the  soft 
tissue  surrounding  the  root  partakes  more  of  the  character 
of  ordinary  fibrous  tissue,  and  may  be  teased  out  nito 
fibrils.  The  fibrou.s  bands  run  mainly  in  a  direction  from 
the  alveolus  towards  the  tooth.  Many  of  them  pass  through 
the  whole  thickness  of  the  soft  structures,  extendincr  from  the 


168  A    MANUAL    OF   DENTAL    ANATOMY. 


bone  of  the  alveolus  to  the  cementum  of  the  tooth,  becoming 
lost  at  each  extremity  in  the  one  tissue  or  other. 

The  osteoblasts  form  both  matrix  and  bone  corpuscles  : 
in  Professor  Klein's  words  "  each  osteoblast  by  the  peripheral 
portion  of  its  cell  substance  gives  origin  to  the  osseous  ground- 
substance,  while  the  central  protoplasm  round  the  nucleus 
persists  with  the  latter  as  the  nucleated  bone-cell.  The  bone- 
cell  and  the  space  in  which  it  lies  become  branched.  For 
a  row  of  osteoblasts  we  then  find  a  row  of  oblong  or  round 
territories,  each  composed  of  matrix,  and  in  it  a  nucleated 
branched  cell.  The  outlines  of  individual  territories  are 
gradually  lost,  and  we  have  then  a  continuous  osseous 
lamina,  with  its  bone-cells.  The  ground  substance  is  from 
the  outset  a  network  of  fibrils  ;  it  is  at  first  soft,  but  soon 
becomes  impregnated  with  inorganic  salts,  the  process  com- 
mencing at  the  '  point  of  ossification.'  The  bone  cells,  with 
their  pi'ocesses,  are  situated  in  corresponding  lacunae  and 
canaliculi,  j  ust  as  in  the  adult  osseous  substance. " 

Thus  jiist  as  calcification  in  an  enamel  cell  or  in  an  odon- 
toblast commences  upon  its  surface,  and  proceeds  inwards 
till  it  has  more  or  less  completely  pervaded  it,  so  in  the  case 
of  the  osteoblast  the  deposition  of  calcareous  salts  proceeds 
from  without  inwards.  To  use  a  rough  comparison  we 
might  imagine  a  calcifying  osteoblast  as  like  an  egg-shell, 
the  central  cavity  of  which  was  being  gradually  obliterated 
by  the  addition  of  successive  layers  on  its  interior  (it  is  not 
to  be  understood  that  any  such  lamination  is  to  be  detected 
in  an  individual  osteoblast).  In  a  certain  number  of  osteo- 
blasts this  process  of  calcification  does  not  proceed  with 
such  regularity  as  to  obliterate  their  centres,  and  at  the 
same  time  to  fuse  together  their  exteriors,  but  as  it  pro- 
gresses with  some  degree  of  irregularity  towards  the  centre, 
tracks  of  uncalcified  matrix  are  left,  and  finally  it  stops 
short  of  obliterating  the  central  portion  of  the  cell.  Al- 
though for  the  purpose  of  description  I  have  spoken  of  the 


THE   DEVELOPMENT    OF    THE    TEETH.  169 


centre  of  the  osteoblast  cell  as  a  '  space,'  of  course  it  is  not 
hollow,  but  consists  of  uncalcified  matrix,  and  in  this  situa- 
tion lies  the  nucleus  of  the  cell. 

In  carmine-stained  preparations  from  the  teeth  of  calves 
a  round  nucleus  may  sometimes  be  seen  lying  in  the  stellate 
"  lacuna  ; "  the  nucleus  soon  disappears,  and  plays  no  active 
part  in  determining  the  form  of  the  lacuna.  The  nucleus 
may  also  be  seen  in  the  developing  bones  of  human  foetuses 
and,  though  this  is  difficult  to  understand,  the  traces  of  the 
nucleus  seem  to  be  beautifully  preserved  in  the  lacunte  of 
a  supposed  Pterodactyle  bone  from  the  Wealden,  a  section 
from  which  was  ligured  by  my  father  in  the  paper  referred 
to.     Exactly  as  calcification,  advancing  with  irregularity  in 

Fig.  76  ('). 


the  interior  of  an  individual  cell,  fails  to  render  it  homo 
geneous  by  pervading  its  whole  substance,  so  it  may  fail  so 
completely  to  unite  contiguous  cells  as  to  obliterate  their 
contours.  A  lacuna,  suiTounded  by  such  a  contour  line, 
mapping  the  limits  of  the  original  cell,  or  cluster  of  cells,  is 
what  is  termed  an  "  encapsuled  lacuna." 

That  Avhich  determines  the  formation  of  a  lacuna,  or  an 
encapsiiled  lacuna,  at  any  particular  spot,  is  unknown  :  all 
that  can  certainly  be  said  upon  the  subject  is  embodied  in 

(')  Encapsuled  laciinse 


170  A    MANUAL    OF   DENTAL    ANAT03IY. 

the  following  extract  from  the  paper  by  my  father  and  Mr. 
De  Morgan,  above  alluded  to  : — "  We  see  the  bomidary  of 
the  original  lacunal  cells  only  in  those  cases  where  the 
lacunar  have  but  few,  or  are  entirely  devoid  of  canaliculi. 
It  would  appear  to  be  a  law,  to  which  there  are  few,  if  any, 
exceptions,  that  when  anastomosis  is  established  between 
adjoining  lacunee,  the  lacunal  cells  blend  with  the  con- 
tiguous parts,  and  are  no  longer  recognisable  as  distinct 
bodies." 

According  to  Kolliker,  the  cemeutum  first  is  deposited  iu 
isolated  scales,  which  coalesce  with  one  another,  rather  than 
in  a  continuous  sheet.  In  the  teeth  of  the  Primates,  the 
Carnivora,  Insectivora,  &c.,  the  cementum,  at  least  in  any 
appreciable  thickness,  is  confined  to  the  roots  of  the  teeth. 
Various  reasons,  however,  exist,  for  regarding  Nasmyth's 
membrane  as  an  exceedingly  thin  layer  of  cement,  which 
have  been  entered  into  in  the  section  relating  to  that 
structure,  and  need  not  be  recapitulated  here.  It  will 
suffice  to  say,  that  it  appears  to  be  one  of  those  structures 
midway  betwixt  full  calcification  and  full  vitality,  and 
shares  with  such  substances  the  power  of  resistance  to 
chemical  reagents  which  characterises  them. 

M.  Magitot  states  that  the  calcification  of  the  cartilaginous 
cement  organ  of  Herbivora  differs  in  no  respect  from  that  of 
other  cartilages,  but  in  his  description  he  merely  states  that 
patches  of  calcification  appear  here  and  there  in  the  deepest 
jjortion  of  the  organ,  coalesce,  and  come  to  invade  its  entire 
thickness ;  and  further  that  the  cement  at  the  period  of 
eruption  is  constituted  of  "  osteoplasts "  regularly  grouped 
round  vascular  canals,  and  included  in  a  ground  substance 
finely  striated.  {Joimial  deVanatomie,  1881,  p.  32.)  Where 
intra-cartilaginous  ossifications  occur  elsewhere  in  the  body 
a  temporary  bone  is  formed  by  the  calcification  of  the 
cartilage  matrix,  which  is  subsequently  absorbed  and  swept 
away,  as  marrow-containing  channels  appear  in  it,  and  bore 


THE   DEVELOPMENT    OF    THE    TEETH.  171 


their  -way  through  it,  substituting  for  the  calcified  cartilage 
a  bone  developed  from  osteoblasts,  and  ultimately  all  remains 
of  the  calcified  cartilage  or  temporary  bone  disappear.  Thus 
all  bone  whether  developed  in  cartilage  or  in  membrane 
is  formed  alike,  the  calcified  cartilage  merely  forming  a 
temporary  framework  or  scaffolding,  in  and  amongst  which 
the  bone  is  formed  from  osteoblasts.  But  M.  Magi  tot  does 
not  describe  in  much  detail  this  calcification  of  cartilage  and 
subsequent  removal  to  give  place  to  an  osteoblast-derived 
bone,  though  he  speaks  of  the  cartilaginous  cement  organ 
as  a  transitory  or  temporary  structure. 

Membrana  Preformativa. — To  the  student  of  dental 
development  few  things  are  more  perplexing  than  the  con- 
flicting statements  wdiich  he  reads  in  various  works  as  to 
the  nature  and  position  of  the  Membrana  preformativa,  of 
which  I  have  hitherto  studiously  avoided  all  description; 
Avhile  it  is  not  encouraging,  after  having  mastered  with 
difiiculty  some  one  description  of  its  character,  to  find  that 
many  of  the  most  recent  authors  altogether  deny  its  exist- 
ence. I  will  endeavour,  therefore,  so  far  as  I  am  able, 
although  not  myself  believing  in  its  presence,  to  put  the 
matter  in  a  clearer  light,  and  to  point  out  wherein  lie  the 
discrepancies  of  statement. 

According  to  the  older  theories  of  tooth  development, 
under  the  thrall  of  which  most  authors  have  written,  the 
tooth  germ  was  in  the  first  instance  a  free,  uncovered  papilla 
of  the  mucous  membrane,  which  subsequently  sank  in  and 
became  encapsulated,  &c.,  &c.,  (see  page  129).  Moreover,  it 
was  taught  by  the  older  histologists  that  fine  homogeneous 
"  basement  membranes  "  were  to  be  found  in  a  great  variety 
of  situations,  amongst  others  beneath  the  epithelium  of  the 
mucous  membrane,  and  that  these  were  of  (physiologically) 
umch  importance,  inasmuch  as  they  formed  defining  limits, 
through  which  structures  did  not  pass.  As  a  necessary  con- 
sequence of  these  views,  it  was   assumed  as  a  matter  of 


172  A    MANUAL    OF   DENTAL    ANATOMY. 


course  that  the  "  dentine  papilla "  was  covered  over  by  a 
"  basement  membrane,"  or  membrana  prcformativa. 

Thus  this  membrane  necessarily  intervened  between  the 
enamel  organ  and  the  dentine  papilla,  and  hence  gave  rise 
to  difficulties  in  the  understanding  of  the  calcifying  process. 
Henle  considered  that  evidences  of  its  presence  speedily 
became  lost,  but  that  ossification  proceeded  in  opposite 
directions  upon  the  two  sides  of  this  membrane :  from 
within  outwards  in  the  case  of  the  enamel,  from  without 
inwards  in  the  case  of  the  dentine. 

Prof.  Huxley,  starting  on  the  same  hypothesis  as  to  its 
position,  namely,  that  it  was  between  the  enamel  organ  and 
the  dentine  papilla,  came  to  a  different  conclusion  as  to  its 
after  fate ;  relying  upon  the  fact  that  a  continuous  sheet  of 
tissue  or  membrane  can  be  raised  from  the  surface  of  the 
developing  enamel  (see  page  152),  he  concluded  that  this 
was  the  original  membrana  preformativa,  that  it  afterwards 
became  the  Nasmyth's  membrane,  and  that  enamel  was 
developed  without  the  direct  participation  of  the  enamel 
organ,  seeing  that  a  membrane  separated  the  two.  My 
reason  for  doubting  the  correctness  of  these  conclusions  has 
been  there  given;  the  membrane  so  demonstrable  is,  I 
believe,  artificial,  and  does  not  represent  any  naturally 
existing  structure. 

KoUiker  strongly  affirms  the  existence  of  the  membrana 
preformativa,  and  in  the  older  edition  of  his  Histology,  held 
that  if  became  converted  into  Nasmyth's  membrane  ;  al- 
though he  now  gives  a  different  explanation  of  the  origin 
of  Nasmyth's  membrane,  I  have  not  found  a  definite  state- 
ment as  to  his  recent  views  of  the  ultimate  fate  of  the 
membrana  preformativa. 

We  have  thus  three  destinations  assigned  to  the  mem- 
brane covering  the  dentine  papilla,  or  membrana  prefor- 
mativa. 


THE    DEVELOPMENT    OF    THE    TEETH.  173 


(i.)  Between  the  dentiue  and  the  enamel  (Henle). 
(ii.)  Between  the  enamel  and  tlie  enamel  organ,  or  out- 
side the  enamel  (Huxley), 
(iii.)  Between  the  dentine  and  the  pulp  (several  writers  of 
less  authority). 

"VVe  come  next  to  those  writers  who  deny  its  existence 
altogether,  explaining  on  other  gi'ounds  the  appearances 
observed. 

Markusen  believed  that  it  was  nothing  more  than  the  part 
of  the  papilla  first  ossified  ;  and  Dr.  Lionel  Beale  definitely 
denies  the  existence  of  a  membrane  in  any  one  of  the  three 
situations  above  detailed,  as  do  also  Hertz,  Wenzel,  and 
Waldeyer. 

Messrs.  Robin  and  Magitot  have  offered  a  plausible  ex- 
planation of  the  appearance  of  a  limiting  membrane  over 
the  pulp,  which  is  briefly  this  :  the  formative  pulp  is  rich 
in  a  clear  substance  of  gelatinous  consistency  (which  in 
fact  forms  its  chief  bulk),  and  which  reminds  the  observer 
of  the  tissue  contained  in  an  umbilical  cord.  This  is  some- 
what more  dense  towards  the  surface,  where  it  forms  a 
matrix  for  the  odontoblasts  and  projects  beyond  them,  so 
as  to  look,  in  section  or  at  a  thin  edge,  like  a  sort  of 
varnish  to  the  papilla.  From  its  greater  density  near  the 
surface,  it  may  become  corrugated,  and  so  look  like  a 
folded  or  torn  membrane.  I  am  quite  inclined  to  agree 
with  the  foregoing  explanation. 

I  am  inclined  to  think,  that  but  for  the  eiToneous  theories 
that  the  dentine  germ  originated  as  a  free  papilla  on  the 
surface,  which  would  according  to  the  prevalent  view  have 
been  necessarily  invested  by  a  basement  membrane,  we 
should  never  have  heard  of  a  membrana  preformativa.  At 
all  events  it  is  difficult  to  imagine  that  such  a  membrane 
exists  upon  papilla  formed  at  such  a  great  distance  from  the 
surface  as  those  of  tlie  snake  or  the  lizard  (P'igs.  61  and  62) ; 


174  A    MANUAL    OF   DENTAL    ANATOMY. 


and  if  there  be  such  a  membrane,  it  must  be  a  secondary 
development  upon  the  surface  of  the  mass  of  cells  -which 
primarily  constitute  the  rudiment  of  the  dentine  papilla,  and 
in  that  case  is  not  a  part  of  the  general  basement  membrane 
of  the  oral  mucous  membi'ane ;  or  else  it  must  have  been 
carried  above  as  a  sort  of  cul  de  sac  in  front  of  the  inward 
growing  process  of  epithelium,  to  which  in  that  case  it  would 
belong  rather  than  to  the  dentine  germ.  Neither  of  these 
suppositions  commend  themselves  as  probable ;  and  a  still 
greater  obstacle  to  the  acceptance  of  a  membrane  in  this 
position  is  afforded  by  the  structure  of  Marsupial  teeth  (see 
fig.  23),  in  which  the  membrane  would  be  everywhere  per- 
forated by  the  soft  contents  of  the  dentine  and  enamel 
tubes. 


Robin  et  Mag-itot.     Journal  de  I'anatomie.     1866. 
Legkos  et  Magitot.     FoUicule  Dentaire.    Journal  de  ranatomie 
de  M.  Ch.  Robin,  1873. 
Morphol.  du  foUicule  dentaire.     1879. 
Fonnation  de  I'organe  dentaire.     1881. 
Klein.    Atlas  of  Histology.     1880. 
Waldeyer.     Strieker's  Histology.     1870. 
Huxley.     Quart.  Jour.  Micros.  Science.     1853. 
Kollikek.     Gewebelehre. 
Tomes,  J.     Quart.  Jour.  Micros.  Science,  1853. 

Dental  Surgery.     1859. 
Tomes,  Chakles  S.     Develop,  of  Vascular  Dentine.     Philos.  Trans. 
1878. 
Develop,  of  Teeth  of  Batrachia.  Ophidia,  Se- 
lacliia,  and  Teleostei.     Phil.  Trans.  1875 — 
187G. 
On   Nasmyth's   Membrane.      Q.   J.    Microsc 
Science,  1872. 
Owen.     Odontography.     1845. 

Anatomy  of  Vertebrates.     1870. 
Nasmyth.     Med.  Chirurg.  Transact.     1839. 

Observations  on  the  Teeth.     1835. 
Mabcxjsen.     Bulletin  de  I'Acad.  de  S.  Petersburg.     1849. 
GOODSIB.     Edinbm-gh  Med.  and  Surg.  Journal.     1838. 
Beale,  Dr.  Lionel.     Structure  of  the  Simple  Tissues.     Archives 

of  Dentistry,  vol.  i. 
Dursy,  EiiiL.     Entwickelungsgeschichte  des  Kopfes.     1869. 


THE   DEVELOPMENT    OF    THE  JAW. 


Heetwig.     Entwickelung  der  Placoidscliuppen  uud  Ziihnc.     Jen- 
aische  Zeitsclirift.     1874. 
Zahnsystem  der  Amphibien.      Ai-cliiv.    f.   Mik.  Anat. 
187i. 
Raschkow.     Meletemata  circa  Dentium  Evohitionem.     1835. 
Heincke.     Zeitsclirift  f.  "Wiss.  Zool.     Bd.  :ixiii.     187:;. 
WeI)L.     Pathologie  der  Zahne.     1870. 
Tomes  axd  De  5lOEaAX.     On  Development  of  Bone.     Phil.  Trans. 

1852. 
Gegenbal'E.     Manual   of  Comparative  jinatomj.     Translated  by 

Jeffrey  Bell.  1878. 
ROLLET.    Connective  Tissues,  in  Strieker's  Histology, 
Haetixg.     Quart.  Journal  Micros.  Science.     1872. 
Eainie.     Brit,  and  Foreign  Med.-Chimrg.  Review.     1S.")7. 
Dean.  31.  S.    Annotated  Translation  of  Robin  and  Magitot  on  the 
Origin  of  the  Dental  Follicle.     Chicago,  1880. 


CHAPTER  V. 

THE  DEVELOPMKXT  OF  THE  JAWS  —  THE  ERUPTION  AND  THE 
ATTACHMENT  OF  THE  TEETH. 

At  an  early  period  in  the  development  of  the  embryo 
there  is  a  single  primitive  bnccal  cavity,  which  is  subse- 
quently divided  into  a  nasal  and  an  oral  cavity  by  the 
palatine  plates  growing  horizontally  across  it ;  the  phaiynx 
l)ehind  the  hinder  end  of  the  primitive  buccal  cavity 
remains  undivided.  Both  upper  and  lower  jaws  make  their 
appearance  about  the  twentieth  day  as  little  buds  from  the 
first  visceral  arch,  and  grow  inwards  towards  the  middle 
line  :  those  which  form  the  lower  jaw  reach  to  the  middle 
and  there  coalesce,  those  for  the  upper  jaw  stop  short,  and 
the  gap  left  between  them  is  filled  by  a  double  downward 
sprouting  process  from  the  forehead,  which  afterwards  forms 
the  intermaxillary  bone.  A  failure  in  the  coalescence  of  the 
maxillary  processes  with  this  intermaxillary  process,  on 
one  or  both  sides,  results  in  a  single  or  double  hare-lip. 

In  the  lower  jaw  or  mandibular  processes  there  ap2:)ears, 
about  the  end  of  the  first  month,  a  dense  cord  of  cartilaginous 
consistence,  Meckel's  cartilage,  which  seems  to  serve  as  a 
scaffolding,  giving  form  and  consistency  to  the  lower  jaw 
prior  to  the  occurrence  of  calcification.  Meckel's  cartilage, 
formed  as  two  distinct  halves,  soon  unites  in  the  middle,  and 
then  forms  a  continuous  curved  bar,  the  hinder  ends  of 
which  reach  up  to  the  tympanum. 

About  the  fortieth  day  a  centre  of  ossification  appears  in 
the  mandibular  process,  which,  spreading  rapidl}',  soon  forms 


THE   DEVELOPMENT    OF    THE   JAJF.  177 


a  slight  osseous  jaw  outside  Meckel's  cartilage,  which  is  not 
however  in  any  way  implicated  in  it,  and  very  soon  begins 
to  waste  away,  so  that  by  the  end  of  the  sixth  month  it  has 
disappeared  :  that  end  of  it  alone  which  extended  up  to  the 
tympanum  does  not  so  waste  away,  but  becomes  ossified  into 
the  malleus.  There  are,  however,  observers  who  hold  that 
in  some  animals,  at  all  events,  Meckel's  cartilage  plays  a 
more  active  part  in  ossification  of  the  jaw. 

In  the  upper  jaw  the  suture  separating  the  intermaxillary 
from  the  maxillary  bones  becomes  obliterated  ver}'  early  on 
the  exterior  surface,  but  it  long  remains  distinguishable  on 
the  palatine  aspect  of  the  bones. 

The  later  changes  which  are  undergone  by  the  jaws  during 
the  development,  eruption,  and  loss  of  the  teeth,  have  long 
engaged  the  attention  of  anatomists,  and  amongst  others  of 
Hunter,  who  was  the  first  to  an-ive  at  a  tolerably  coiTect 
appreciation  of  the  process.  In  the  first  edition  of  my 
father's  "  Dental  Surgery,"  the  results  of  a  very  extensive 
series  of  observations  can-ied  out  upon  maxillae  collected  by 
himself,  were  detailed,  confirming  in  the  main  Hunter's 
conclusions,  but  adding  many  new  points  to  orn*  knowledge  ; 
and  from  this  work  I  have  borrowed  largely  in  the  present 
chapter.  Professor  Humphrey,  who  had  overlooked  these 
descriptions,  which  were  never  published  in  any  other  form 
than  as  an  introduction  to  the  "  Dental  Surgery,"  instituted 
a  series  of  experiments  upon  growing  animals,  which  tended 
towai'ds  the  same  conclusions. 

As  a  means  of  giving  the  student  a  guide  in  his  reading 
of  the  following  pages,  and  a  clue  to  the  results  towards 
which  he  is  being  led,  a  preliminary  statement,  which  does 
not  pretend  to  scientific  accuracy,  may  perhaps  be  useful ; 
while  the  description  given  will  relate  for  the  most  part  to 
the  lower  jaw,  because  its  isolated  position,  bringing  it  into 
relation  with  fewer  other  bones,  renders  it  more  easy  to 
study ;  not  that   any  difference  of  princiiile   underlies  the 


178  A    MANUAL    OF   DENTAL    ANATOMY. 


growth  of  the  upper  jaw.  The  different  parts  of  the  lower 
jaw  answer  for  different  purposes  ;  one  division  of  its  body 
having  a  very  close  and  intimate  relation  with  the  teeth, 
the  other  serving  a  distinct  purpose,  and  being  only  secon- 
darily connected  with  the  teeth, 
^  The  alveolar  portion  of  the  jaw,  that  which  lies  above  the 
level  of  the  inferior  dental  canal,  is  developed  around  the  milk 
teeth  :  when  they  are  lost,  it  disappears,  to  be  re-formed 
again  for  the  second  set  of  teeth,  and  is  finally  wholly 
removed  after  the  loss  of  the  teeth  in  old  age. 

The  portion  of  jaw  below  this  line,  which  is  essential  to 
deglutition  and  respiration,  is  late  in  acquiring  any  con- 
siderable development.  Once  formed  it  is  never  removed, 
save  that  when  in  advanced  old  age  the  muscles  of  mastica- 
tion are  no  longer  in  full  use,  it  becomes,  to  a  slight  extent 
only,  wasted. 

In  order  to  understand  the  drift  of  the  following  descrip- 
tion, it  is  essential  to  keep  in  view  the  different  life  histories 
of  those  two  parts  of  the  jaw  just  alluded  to. 

In  an  early  foetus,  long  before  the  necessity  for  respiratory 
movement  or  deglutition  has  become  imminent,  a  thin  lamina 
of  bone  has  begun  to  be  formed  beneath  the  tooth  germs, 
forming,  as  it  were,  a  semicircular  gutter  running  round  the 
jaw,  in  which  the  developing  tooth  sacs  are  lodged.  The 
thin  gutter  of  bone  thus  formed  is  above  and  outside 
Meckel's  cartilage,  and  intervenes  between  the  rudimentary 
inferior  maxillary  vessels  and  nerves,  and  the  teeth.  The 
sides  of  the  bony  furrow  rise  as  high  as  the  top  of  the  tooth 
germs,  but  they  do  not  arch  over  and  cover  them  in,  in  such 
manner  as  the  permanent  tooth  germs  are  arched  in,  for  the 
long  fuiTOW  is  widely  open  at  the  top. 

Passing  on  to  the  condition  of  the  mandibles  at  the  time 
of  birth,  the  two  halves  are  as  yet  not  anchylosed,  but  are 
united  only  by  fibro-cartilage.  "  The  alveolar  margins  are 
deeply  indented  with  lai-ge  open  ciypts,  more  or  less  per- 


THE   DEVELOPMENT    OF    THE    JAW 


179 


fectly  formed.     The  depth  of  these  bony  cells  is  only  suffi- 
cient to  contain  the  developing  teeth   and  tooth  pulps,  the 


Fig.  77  ('). 


(')  Upper  and  lower  jaws  of  a  nine  months  fcetus,  the  teeth  having 
been  removed  from  the  jaws  on  one  side  to  show  the  extent  to  which  they 
are  calcified  at  this  period.  (Two-thirds  life  size. )  a.  Alveoli  of  lateral 
incisors,  h.  Alveoli  of  canines,  c.  Alveoli  of  second  temporary  and  first 
permanent  molars.  A  bristle  has  been  passed  through  the  inferior  dental 
canal. 

N  2 


180  A    MANUAL    OF   DENTAL    ANATOMY. 


former  rising  to  the  level  of  the  alveolar  margins  of  the 
jaws.  At  this  period  the  crypts  or  alveoli  are  not  arranged 
in  a  perfectly  uniform  line,  nor  are  they  all  equally  complete. 
The  septa,  which  divide  into  a  sei'ies  of  cells  that  which  at 
an  earlier  nge  was  but  a  continuous  groove,  are  less  perfect 
at  the  back  than  at  the  front  part  of  the  mouth.  The 
alveoli  of  the  central  incisors  of  the  upper  and  the  lower 
jaws  are  a  little  larger  within  than  at  the  orifice,  and  this 
difference  is  made  still  greater  by  a  depression  upon  the 
lingual  wall  of  each  for  the  reception  of  the  pulp  of  the 
corresponding  permanent  tooth.  They  are  divided  from 
the  crypts  of  the  lateral  incisors  by  a  septum  which  runs 
obliquely  backwards  and  inwards  towards  the  median  line. 
The  sockets  for  the  lateral  incisors  occupy  a  position  slightly 
posterior  to  those  for  the  central  teeth,  and  are  divided 
from  the  canine  alveoli  by  a  septum  which  proceeds  obliquely 
backwards,  and  in  the  lower  jaw,  as  regards  the  median  line 
of  the  mouth,  outwards.  By  the  arrangement  of  these 
divisions  the  alveoli  of  the  central  incisors  are  rendered 
broader  in  front  than  behind,  and  the  relative  dimensions 
of  the  sockets  of  the  lateral  teeth  are  reversed,  as  shown  in 
Fig.  77.  The  crypts  of  the  canine  teeth  are  placed  a  little 
anteriorly  to  those  of  the  laterals,  and  nearly  in  a  line  with 
those  of  the  central  incisors,  giving  to  the  jaws  a  somewhat 
flattened  anterior  aspect." 

While  the  main  bulk  of  the  lower  jaw  is  made  up  by  the 
alveoli  of  the  teeth,  in  the  upper  jaw  the  alveoli  descend 
but  little  below  the  level  of  the  palatal  plates,  though  the 
sockets  are  tolerably  deep.  The  antrum  as  a  special  distinct 
cavity  cannot  be  said  to  exist,  being  merely  represented  by 
a  depression  upon  the  wall  of  the  nasal  cavity,  the  alveolar 
cavities  therefore  being  separated  only  by  a  thin  plate  of 
bone  from  the  orbits. 

The  figure  represents  also  the  extent  to  which  calcification 
has  advanced  in  the  various  teeth. 


THE   DEVELOPMENT    OF    THE   JAW.  181 


A  full  half  of  the  length  of  the  crowns  of  the  central 
incisors,  about  half  that  of  the  laterals,  and  the  tips  only  of 
the  canines  are  calcified ;  the  first  temporary  molars  are 
complete  as  to  their  masticating  surfaces  ;  the  second  tem- 
porary molars  have  their  cusps  more  or  less  irregularly 
united,  in  many  specimens  the  four  cusps  being  united  into 
a  ring  of  dentine,  the  dentine  in  the  central  depression  of 
the  crown  not  being  yet  formed.  During  the  formation  of 
the  permanent  teeth,  very  similar  relations  exist  between 
the  amount  of  calcification  in  the  incisors  and  canines  ;  thus 
when,  as  sometimes  happens,  the  development  of  the  teeth 
proceeds  very  imperfectly  up  to  a  certain  date,  and  then 
changes  for  the  better,  it  may  be  that  the  lower  half  of  the 
crown  of  the  central  incisor,  somewhat  less  of  the  lateral, 
and  the  extreme  tip  of  the  canine  will  be  honeycombed, 
■while  the  remainder  of  the  tooth  will  be  perfect,  thus  per- 
petuating an  evidence  of  the  stages  to  which  each  of  these 
teeth  had  at  that  particular  period  attained. 

Having  noted  in  some  detail  the  characters  of  the  jaws  of 
a  nine  months  foetus,  we  may  pass  on  to  the  consideration 
of  those  changes  which  precede  the  cutting  of  the  deciduous 
teeth.  A  general  increase  in  size  takes  place,  new  bone 
being  developed  at  all  those  points  where  the  maxillse  are 
connected  by  soft  tissue  with  other  bones,  as  well  as  from 
their  own  periosteum.  But  the  increase  in  dimensions  does 
not  take  place  in  all  directions  equally,  so  that  material 
changes  of  form  result. 

In  con-espondence  with  the  elongation  of  the  tooth  sacs, 
the  alveoli  become  increased  in  depth,  and  their  edges  circle 
inwards  over  the  tooth  sacs ;  active  development  of  bone 
takes  place  in  the  sutvu-es  uniting  the  two  halves  of  the  jaws 
to  one  another,  which  is  compensated  by  the  inclination 
in  wards, of  the  alveoli  of  the  central  incisors.  In  the  lower 
jaw  the  articidar  process,  at  first  hardly  raised  above  the 
level  of  the    alveolar  border,  rises  rapidly  up,  the  direction 


182  A    MANUAL    OF   DENTAL    ANATOMY. 


of  the  ramus  at  first  remaining  oblique,  though  the  angle  of 
the  jaw  becomes  developed  as  a  stout  process  for  the  attach- 
ment of  muscles.  At  the  age  of  six  months  the  symphysis 
is  still  well  marked,  and  the  mental  prominence  first  becomes 
noticeable. 

Fig.  78  ('). 


An  additional  bony  crypt  for  the  first  permanent  molar 
has  also  appeared,  though  its  separation  from  that  of  the 
second  temporary  molar,  from  which  it  was  at  first  in  no 
way  distinct,  is  yet  incomplete,  especially  in  the  lower  jaw. 
In  the  upper  jaw  the  first  permanent  molar  crypt  has  no 
posterior  wall ;  bony  cells  for  the  permanent  central  incisors 
are  well  marked,  but  those  for  the  laterals  are  mere  deep 
pits  in  the  palatine  wall  of  the  crypts  of  the  temporary  teeth. 

At  the  age  of  eight  months,  or  thereabouts,  the  process 
of  the  eruption  of  the  teeth,  or  "teething,"  has  fairly  set  in  ; 
anchylosis  has  taken  place  at  the  symphysis  of  the  lower 
jaw,  the  mental  prominence  is  well  marked,  and  in  the  upper 
jaw  the  antrum  has  become  a  deep  depression,  extending 
under  the  inner  two-thirds  of  the  orbit. 

Postponing  for  the  moment  the  consideration  of  the  erup- 
tion of  the  teeth,  in  order  to  follow  up  the  growth  of  the 
jaws,  it  becomes  necessary  to  take  some  fixed  points  as 
standards  from  which  to  measure  the  relative  alteration  of 
other  portions  of  the  bone.  In  most  bones,  processes  for 
the  attachment  of  muscles  would  be  very  unsuitable  for  the 
pui-pose,  because  they  would  alter  with  the  general  altera- 

(1)  Lower  jaw  of  a  nine  months  foetus. 


THE   DEVELOPMENT    OF    THE  JAW.  183 

tion  in  the  dimensions  of  the  bone  :  thus  a  process  situated 
at  a  point  oue-third  distant  from  the  articular  extremity  of 
a  large  bone,  will  still  be  found  one-third  distant  from  the 
end,  though  the  bone  has  doubled  in  length.  The  four 
little  tubercles  which  give  attachment  to  the  genio-hyo- 
glossus  and  genio-hyoid  muscles  are  not,  however,  open  to 
these  objections,  as  they  are  already,  so  to  speak,  at  the  end 
of  the  bone,  or,  at  least,  of  each  half  of  it ;  and  their  general 
correspondence  in  level  with  the  inferior  dental  canal,  which 
can  hardly  be  imagined  to  undergo  much  alteration,  in- 
dicates that  their  position  is  tolerably  constant. 

The  points  selected  as  landmarks  are  then,  the  spinte 
mentales,  the  inferior  dental  canal  and  its  orifice,  and  the 
mental  foramen.  The  mental  foramen  itself  does  undergo 
slight  change  in  position,  but  this  change  can  easily  be 
estimated,  and  may  as  well  at  once  be  mentioned.  As  the 
jaw  undergoes  increase  in  size,  large  additions  are  made  to 
its  surface  by  deposition  of  bone  from  the  periosteum,  neces- 
sarily lengthening  the  canal.  The  additions  to  the  canal 
do  not,  however,  take  place  quite  in  the  line  of  its  original 
course,  but  in  this  added  portion  it  is  bent  a  little  outwards 
and  upwards.  If  we  rasp  off  the  bone  of  an  adult  jaw  down 
to  the  level  of  this  bend,  a  process  which  nature  in  great 
part  performs  for  us  in  an  aged  jaw,  or  if  instead  we  make 
due  allowance  for  the  alteration,  the  mental  foramen  becomes 
an  available  fixed  point  for  measurement. 

The  mental  foramen,  which  undergoes  most  of  its  total 
change  of  position  within  a  few  months  after  birth,  comes 
to  correspond  with  the  centre  of  the  socket  of  the  first  tem- 
porary molar ;  later  on  it  corresponds  with  the  root  of  the 
first  bicuspid,  which  is  thus  shown  to  succeed,  in  exact 
vertical  position,  the  first  temporary  molar. 

On  the  inner  surface  of  the  jaw  the  tubercles  for  the 
attachment  of  the  genio-hyo-glossus  and  genio-hyoid  muscles 
ai-e  in  the  foetus,  opposite  to,  and  very  little  below  the  base 


]S4  A    MANUAL    OF   DENTAL    ANATOMY. 


of  the  alveoli  of  the  central  incisors,  a  position  which  they 
afterwards  hold  with  regard  to  the  permanent  incisors.  The 
upper  of  the  two  pairs  of  processes  are  about  at  the  same 
general  level  as  the  mental  foramen. 

The  genei-al  result  arrived  at  by  measurements  taken 
from  these  fixed  points  is  that  the  alveolar  arch  occupied 
by  the  teeth  which  have  had  deciduous  predecessors,  namely 
the  incisors,  canines,  and  bicuspids,  corresponds  very  closely 
with  the  whole  alveolar  arch  of  the  child  in  whom  the  tem- 
porary dentition  is  complete ;  and  that  the  difterences  which 
do  exist  are  referable,  not  to  any  fundamental  alteration  in 
form  or  interstitial  growth,  but  to  mere  addition  to  its 
exterior  sm'face.  Or  more  briefly,  that  the  front  twenty  of 
the  permanent  succeed  vertically  to  the  places  of  the  tem- 
porary teeth,  the  increase  in  the  size  of  the  jaw  in  an  adult 
being  due  to  additions  at  the  back,  in  the  situation  of  the 
true  molars,  and  to  other  points  on  the  surface. 

If  measurements  be  taken  across  between  the  inner  plates 
of  the  alveoli  on  either  side  at  the  points  where  they  are 
joined  by  the  septa  between  the  first  and  second  temporary 
molars,  and  at  about  the  level  of  the  genio-hyo-glossus 
tubercles,  it  will  be  found  that  the  increase  is  slight,  if  any, 
notwithstanding  that  in  other  dimensions  there  is  a  very 
great  difl:erence  between  the  jaws  of  a  nine  months  foetus 
and  of  a  nine  months  child. 

Again,  if  an  imaginary  line  be  sti'etched  across  between 
these  two  points,  and  from  its  centre  a  line  be  drawn  for- 
wards to  the  spina  mentalis  in  the  same  two  jaws,  this  will 
be  found  to  differ  but  little  in  length  in  the  two  specimens. 

But,  if  instead  of  measuring  to  the  spina  mentalis,  the 
line  had  been  carried  to  the  anterior  alveolar  plate,  a  great 
dift'crence  would  have  been  observable  ;  in  point  of  fact,  con- 
temporaneously with  the  development  of  the  crypts  of  the 
permanent  teeth  inside  them,  the  temporary  teeth  and  their 
outer  alveolar  plates  are  slowly  pushed  outwards,  a  process. 


THE   DEVELOPMENT    OF   THE   JAJF. 


185 


the  results  of  which  we  see  in  the  separation  which  cc 
about  between  each  one  of  the  temporary  teeth,  prior  to 
their  being  shed,  where  the  process  of  dentition  is  being 
carried  on  in  a  perfectly  normal  manner. 

Measurements  taken  for  the  sake  of  comparing  adult  jaws 


with  those  of  an  eight  months  child,  give  closely  similar 
results,  which  I  have  endeavoured  to  roughly  embody  in 
the  accompanying  figures. 

In  these  it  is  shown  that  the  increase  in  the  dimensions 
of  the  jaw  has  taken  place  in  two  directions;  by  prolongation 
backwards  of  its  cornua  concomitantly  with  the  addition  at 
the  back  of  the  series  of  teeth  of  the  true  molars,  which  follow 


(')  Diagram  representing  a  jaw  of  a  nine  months  fcetus,  superimposed 
upon  an  adult  jaw,  to  show  in  what  directions  increase  has  taken  place. 


186  A    MANUAL    OF  DENTAL    ANATOMY. 


one  another  at  considerable  intervals  ;  and  by  additions  to 
its  exterior  surfoce  by  which  it  is  thickened  and  strengthened. 
The  study  of  the  growth  of  the  jaw  in  vertical  depth  is  also 
very  instructive.  We  find  that,  as  has  already  been  men- 
tioned, the  history  of  that  part  of  the  jaw  which  lies  below 
the  inferior  dental  canal  is  very  different  from  that  which 
lies  above.  From  the  time  of  birth  to  that  at  which  the 
temporary  teeth  begin  to  be  cut,  the  jaw  below  that  line 
has  been  making  steady  but  slow  progress  in  vertical  depth ; 
the  alveoli,  above  that  line,  have  been  far  more  active  but 
far  more  intermittent  in  their  development. 

Again,  passing  from  the  nine  months  fcetus  to  the  seven 
years  old  child,  in  whom  the  temporary  dentition  is  complete, 
the  framework  of  the  jaw  below  our   imaginary  line  has 


far/t/.f  _p/77c>/!///j 


attained  to  a  depth  almost  equal  to  that  which  it  is  seen  to 
have  in  an  adult ;  in  the  adult  again  it  corresponds  pretty 
well  with  that  in  an  aged  jaw.  The  alveolar  portion,  how- 
ever, is  far  deeper  in  the  adult  than  in  the  child  (this 
difference  is  not  sufficiently  well  marked  in  the  figure),  and 

(1)  Lower  jaw.     The  horizontal  line  marks  the  level  of  the  inferior 
dental  canal. 


THE   DEVELOPMENT    OF    THE   JAW. 


187 


in  fact  constitutes  almost  the  whole  increase  in  vertical 
dimensions  down  the  passage  from  the  child's  to  the  adult's 
form  of  the  jaw. 

Fig.  81  ('). 


In  the  lower  jaw  we  may  take  it  as  i^roven  that  the 
basal  portion  has  little  relation  to  the  development  of 
the  teeth,  but  that  the  alveolar,  or  upper  portion  is  in 
entire   and   absolute   dependence   upon    them,    a   point   to 


(^)  Lower  jaw  of  an  adult. 

('-)  Lower  jaw  of  an  aged  person,  the  dotted  lines  indicating  the  outline 
of  the  parts  removed  by  absorption,  as  the  jaw  assumes  the  form  charac- 
teristic of  advanced  age. 


188  A    MANUAL    OF  DENTAL    ANATOMY. 


which  I  shall  again  return  in  speaking  of  the  erui)tion  of 
the  teeth. 

It  remains  to  speak  in  some  further  detail  of  the 
precise  means  by  which  the  enlargement  of  the  jaw  is 
effected. 

To  a  slight  extent  there  is  formation  of  bone  going  on  at 
the  symphysis,  prior  to  the  comi^lete  anchylosis  taking 
place  :  the  share  taken  by  this  in  increasing  the  size  of  the 
jaw  would,  however,  appear  to  be  but  small,  after  the  ter- 
mination of  the  intra-uterine  period.  Additions  to  the 
surface,  at  the  edges  of  the  alveoli  and  at  the  base  of  the 
jaw  are  continually  going  on,  and  bring  about  that  addition 
to  the  exterior  already  noticed. 

But  the  main  increase  in  the  size  of  the  jaw  has  been  in 
the  direction  of  backward  elongation ;  in  this,  as  Kolliker 
first  pointed  out,  the  thick  articular  cartilage  plays  an  im- 
portant part.  The  manner  in  which  the  jaw  is  formed 
might  almost  be  described  as  wasteful ;  a  very  large  amount 
of  bone  is  formed  which  is  subsequently,  at  no  distant  date, 
removed  again  by  absorption;  or  we  might  compare  it  to  a 
modelling  process,  in  which  thick,  comparatively  shapeless 
masses,  are  dabbed  on  to  be  trimmed  and  pared  down  into 
form. 

To  bring  it  more  clearly  home  to  the  student's  mind,  if 
all  the  bone  ever  formed  were  to  remain,  the  coronoid  pro- 
cess would  extend  from  the  condyle  to  the  region  of  the  first 
bicuspid,  and  all  the  teeth  behind  that  would  be  buried  in 
its  base  :  there  would  be  no  "  neck "  beneath  the  condyle, 
but  the  internal  oblique  line  would  be  a  thick  bar,  corre- 
sponding in  width  with  the  condyle.  It  is  necessary  to 
fully  realise  that  the  articular  surface  with  its  cartilage 
has  successively  occupied  every  spot  along  this  Ihie  ;  and  as 
it  progresses  backwards  by  the  deposition  of  fresh  bone  in 
its  cartilage,  it  has  been  followed  up  by  the  process  of 
absorption  removing  all  that  Avas  redundant. 


THE   DEVELOPMENT    OF    THE   JAW. 


On  the  outer  surface  of  the  jaw  we  can  frequently  discern 
a  shght  ridge,  extending  a  short  distance  from  the  head  of 
the  bone  ;  but  if  the  prominence  were  preserved  on  the 
inner  surface,  the  inferior  dental  artery  and  nerve  would  be 
turned  out  of  their  course.  We  have  thus  a  speedy  removal 
of  the  newly-formed  bone,  so  that  a  concavity  lies  imme- 
diately on  the  inner  side  of  the  condyle  ;  and  microscopic 
examination  of  the  bone  at  this  point  shows  that  the  lacuna3 
of  Howship,  those  characteristic  evidences  of  absorption, 
abundantly  cover  its  surface,  showing  that  here  at  least 
absorption  is  most  actively  going  on. 

In  the  same  way  the  coronoid  process,  beneath  the  base 
of  which  the  first,  second,  and  third  molars  have  successively 
been  formed,  has  moved  backwards  by  absorption  acting  on 
its  anterior,  and  deposition  on  its  posterior  surfaces. 

The  periosteum  covering  the  back  of  the  jaw  is  also  active 
in  forming  the  angle  and  the  parts  thereabouts. 

It  is  worth  while  to  add  that  the  direction  of  growth  in 
young  jaws  is  marked  by  a  series  of  minute  ridges  ;  in  like 
manner  the  characteristic  marks  of  absorption  are  to  be 
found  about  the  neck  of  the  condyle,  and  the  front  of  the 
coronoid  process,  and  those  of  active  addition  about  the  pos- 
terior border,  so  that  the  above  statements  I'est  upon  a  basis 
of  observation,  and  are  not  merely  theoretical.  Two  cases  of 
arrested  development  of  the  jaw  ("Dental  Surgery,"  p.  108) 
lend  a  species  of  experimental  proof  to  the  theory  of  the 
formation  and  growth  of  the  jaw  above  given. 

There  are  authors,  however,  who  maintain  that  the  growth 
of  the  jaws  is  not  merely  a  backward  elongation  of  the 
cornua,  together  with  additions  to  the  external  surface,  but 
that  an  "  interstitial  growth"  takes  place. 

AVedl  inclines  to  this  latter  view,  and  the  question  cannot, 
I  think,  be  held  to  be  absolutely  settled.  Although  it  is 
difficult  to  form  any  definite  conception  of  interstitial  growth 
in  a  tissue  so  dense  and  unvielding  as  bone,  so  that  the  doc- 


190  A    MANUAL    OF  DENTAL    ANATOMY. 

trines  promulgated  in  the  foregoing  pages  have  the  support 
of  ct.  priori  probability,  there  are  some  rather  paradoxical 
facts  to  be  met  with  in  comparative  odontology.  Never- 
theless, there  can  be  no  doubt,  that  backward  elongation  as 
teeth  are  successively  added,  &c.,  is  sufficiently  near  the 
truth  in  the  case  of  human  and  most  mammalian  jaws  for 
practical  jDurposes. 

It  remains  to  notice  the  changes  in  form  which  the 
ascending  ramus  and  the  angle  of  the  jaw  undergo.  In 
the  foetus  the  ramus  is  but  little  out  of  the  line  of  the  body 
of  the  jaw,  and  the  condyle  little  raised  above  the  alveolar 
border. 

Gradually  the  line  of  development,  as  is  indicated  even  in 
the  adult  jaw  by  the  course  of  the  inferior  dental  canal,  takes 
a  more  upward  direction ;  copious  additions  of  bone  are 
made  on  the  posterior  border  and  about  the  angle,  so  that  in 
an  adult  the  ramus  ascends  nearly  at  right  angles  to  the  body 
of  the  jaw. 

In  old  age,  concomitantly  with  the  diminution  of  muscular 
energy,  the  bone  about  the  angle  wastes,  so  that  once  more 
the  ramus  appears  to  meet  the  body  at  an  obtuse  angle. 
But  all  the  changes  which  mark  an  aged  jaw  are  the  simple 
results  of  a  supex'ficial  and  not  an  interstitial  absorption,  cor- 
responding with  a  wasting  of  the  muscles,  of  the  pterygoid 
plates  of  the  sphenoid  bone,  &c. 


ERUPTION    OF    THE    TEETH. 


The  mechanism  by  which  teeth,  at  the  date  of  eruption, 
are  pushed  upwards  into  place,  is  far  from  being  perfectly 
understood.  The  simplest  theoiy  would  appear  to  be  that 
they  rise  up,  in  consequence  of  the  addition  of  dentine  to 
their  base  ;  in  fact,  that  their  eruption  is  due  to  the  elonga- 
tion of  their  fangs. 


ERUPTION   OF    THE    TEETH.  191 

Various  very  strong  objections  have  been  brought  forward, 
clearly  proving  that  this  cause  is  quite  inadequate  to  explain 
all  that  may  be  observed.  In  the  first  place,  teeth  with  very 
stunted  roots — which  may  be  practically  said  to  have  no 
root — are  often  ervipted.  Again,  a  tooth  may  have  the 
whole  length  of  its  roots  completed,  and  yet  remain  buried 
in  the  jaw  through  half  a  person's  life,  and  then,  late  in  life, 
be  erupted.  Moreover,  when  a  healthy  normal  tooth  is  being 
erupted,  the  distance  travelled  by  its  crown  materially  ex- 
ceeds the  amount  of  addition  to  the  length  of  its  roots  which 
has  gone  on  during  the  same  time. 

To  turn  to  comparative  anatomy,  the  tooth  of  a  crocodile 
moves  upwards,  tooth  pulp  and  all,  obviously  impelled  by 
something  different  from  mere  elongation;  and  my  own 
researches  upon  the  development  and  succession  of  rep- 
tilian teeth  clearly  show  that  a  force  quite  independent  of 
increase  in  their  length  shifts  the  position  of,  and  "  erupts" 
successive  teeth.  But  what  the  exact  nature  of  the  impulse 
may  be,  is  an  unsolved  riddle :  the  explanations  which  I  have 
read,  being,  to  my  mind,  less  satisfying  than  the  admission 
that  we  do  not  know. 

Towards  the  eighth  month  of  childhood  the  bony  crypts 
which  contain  the  temporary  teeth  in  the  front  of  the  mouth 
begin  to  be  renewed.  The  process  of  absoi-ption  goes  on 
with  greater  activity  over  the  fronts  of  the  crowns  than  over 
their  apices,  so  that  almost  the  whole  outer  wall  of  the 
alveoli  is  removed.  At  the  back  of  the  mouth  the  crypts 
still  retain  their  inverted  edges  ;  indeed,  development  of  the 
crypts  is  still  going  on  in  this  part  of  the  mouth. 

When  a  tooth  is  about  to  be  cut,  very  active  absorption  of 
its  bony  surroundings  goes  on,  particularly  on  the  anterior 
surface,  the  bone  behind  it  Ijeing  still  i-equired  as  forming 
part  of  the  crypt  of  the  developing  successional  tooth.  But 
no  sooner  has  the  crown  passed  up  through  the  very  wide 
and  fi-ee  orifice  so  formed,  than  absorption  gives  place  to 


192 


A    MANUAL    OF   DENTAL    ANATOMY. 


deposition,  and  the  bone  rapidly  developes  so  as  to  loosely 
embrace  the  neck  of  the  tooth. 

Additions  to  the  margin  of  the  alveoli  keep  pace  with 

Fig.  83  ('). 


the  gradual  elongation  of  the  roots  of  the  teeth ;  as  this  is 
a  moderately  rapid  process,  the  alveolar  portion  of  the  jaw 
increases  in  depth  almost  abruptly. 

But  it  does  not  do  so  iiniformly  all  over  the  mouth  ;  if  it 
did,  the  teeth  could  only  be  closed  at  the  back  of  the  mouth, 
unless  the  rami  elongated  by  an  equally  sudden  accession  of 
new  bone. 

The  front  teeth  are  erupted  first,  and  the  jaw  deepens 
first  in  front  :  later  on  the  back  teeth  come  iip  and  the  jaw 
is  deepened  posteriorly  ;  meanwhile  the  elongation  of  the 


Q)  Jaws  of  a  male  nine  montlis  old,  in  wliicla  the  eruption  of  the  teeth 
is  jnst  commencing. 


ERUPTION    OF  THE    TEETH.  193 

rami  has  been  going  on  slowly,  but  -without  interruption. 
Thus  is  brought  about  a  condition  of  parts  allowing  of 
the  whole  series  of  teeth  coming  into  their  proper  mutual 
antagonism. 

It  was  pointed  out  by  Trousseau  that  the  eruption  of  the 
teeth  is  not  a  contimious  process,  which,  once  commenced,  is 
carried  on  without  intermission  to  its  completion,  but  that  it 
is  interrupted  by  period?  of  repose.  The  teeth  are,  according 
to  his  statement,  cut  in  groups ;  the  eruption  of  the  teeth  of 
each  group  being  rapid,  and  being  succeeded  by  a  complete 
cessation  of  the  process.  Individual  variations  are  nume- 
rous ;  the  following  may  lie  taken  as  an  approximation  to 
the  tiiith  : — 

The  lower  centrals  are  erupted  at  an  age  ranging  from  six 
to  nine  months  ;  their  eruption  is  rapid,  and  is  completed  in 
ten  days  or  thereabouts  ;  then  follows  a  rest  of  two  or  three 
months. 

Next  come  the  four  upper  incisors  ;  a  rest  of  a  few 
months ;  the  lower  laterals  and  the  four  first  molars  ;  then 
a  rest  of  four  or  five  months. 

The  canines  are  peculiar  in  being  the  only  teeth  of  the 
temporary  set  which  come  down  between  teeth  already  in 
place.  To  this,  as  well  as  to  the  greater  length  of  their 
root  (though  it  is  not  quite  clear  what  this  has  to  do  with 
it),  Trousseau  ascribes  the  great  length  of  time  which  their 
eruption  occupies,  it  taking  two  or  three  months  for  its  com- 
pletion. According  to  him,  children  suffer  more  severely 
from  constitutional  disturbance  during  the  cutting  of  these 
teeth  than  that  of  any  other,  but  Dr.  West  thinks  that  the 
eruption  of  the  first  molars  causes  the  most  suffering.  It 
may  also  be  noted  that  the  canines  during  their  develop- 
ment lie  farther  from  the  alveolar  border  than  do  the  other 
teeth,  so  that  they  travel  a  greater  distance  ;  obviously,  not 
merely  from  the  elongation  of  the  root,  which  is  wholly 
inadequate  to  effect  such  a  change  in  position. 


194 


A    MANUAL    OF   DENTAL    ANATOMY. 


The  dates  of  the  eruption  of  the  milk  teeth  vary  much, 
no  two  authors  giving  them  alike ;  but  the  whole  of  the 
deciduous  teeth  are  usually  cut  by  the  completion  of  the 
second  year.  Cases  in  which  incisors  have  been  erupted 
before  birth  are  not  very  uncommon.  At  a  time  when  the 
crowns  of  all  the  deciduous  teeth  have  been  fully  erupted, 
their  roots  are  still  incomplete,  and  are  widely  open  at  their 
bases,  so  that  it  is  not  till  between  the  fourth  and  sixth 
years  that  the  temporary  set  of  teeth  can  be  called  abso- 
lutely complete. 

At  the  sixth  year,  preparatory  to  the  appearance  of  any 


of  the  permanent  teeth,  the  temporary  teeth  maybe  observed 
to  be  slightly  separated  from  each  other ;  they  have  come  to 
occupy  a  more  anterior  position,  pushed  forward,  it  may  be, 
by  the  great  increase  in  size  of  the  crypt  of  the  permanent 
teeth  behind  them.     The  general  relation  of  these  to  the 


(^)  Normal  well-formed  jaws,  from  wliicli  the  alveolar  plate  has  been  in 
great  part  removed,  so  as  to  expose  the  developing  permanent  teeth  in 
their  crypts  in  the  jaws. 


ERUPTION    OF    THE    TEETH.  195 

temporan-  teeth  may  be  gathered  from  the  accompanying 
figure,  in  which  it  "will  be  noticed  that  the  canines  lie  far 
above  and  altogether  out  of  the  line  of  the  other  teeth,  and 
that  a  slight  degree  of  overlapping  of  the  edges  of  the  per- 
manent central  and  lateral  incisors  exists. 

The  bicuspids  lie  in  bony  cells  which  are  embraced  pretty 
closely  by  the  roots  of  the  temporary  molars,  and  it  hence 
happens  that  extraction  of  the  latter  sometimes  brings  them 
away  in  their  entirety. 

The  first  permanent  molars  are  erupted  in  a  manner  closely 
similar  to  that  described  as  occurring  with  the  temporary 
teeth;  that  is  to  say,  their  bony  crypts  become  widely 
opened  out  by  absorption,  the  crown  passes  out,  and  new 
bone  is  rapidly  formed  which  embraces  the  neck,  prior  to 
any  considerable  length  of  root  being  formed. 

Last,  then,  follows  the  absorption  of  the  root  of  the  tem- 
porary teeth,  a  matter  first  accurately  investigated  by  my 
father.  The  root  at  or  near  to  its  end,  becomes  excavated 
by  shallow  cup-shaped  depressions ;  these  deepen,  coalesce, 
and  thus  gradually  the  whole  is  eaten  away.  Although  ab- 
sorption usually  commences  on  that  side  of  the  root  which 
is  nearest  to  the  successional  tooth,  it  by  no  means  invari- 
ably does  so  ;  it  may  be,  and  often  is,  attacked  on  the  oppo- 
site side,  and  in  many  places  at  once. 

The  cementum  is  usiially  attacked  first,  but  eventually 
dentine,  and  even  enamel  come  to  be  scooped  out  and 
removed  by  an  extension  of  the  process.  That  part  of  the 
dentine,  however,  which  immediately  surrounds  the  pulp 
appears  to  have  more  power  of  resistance  than  any  other 
part  of  the  tooth,  and  thus  often  persists  for  a  time  as  a  sort 
of  hollow  column.  The  absorption  of  the  temporary  teeth 
is  absolutely  independent  of  pressure ;  the  varying  position 
of  the  excavation  has  already  been  noticed,  and  it  may  be 
added  that  in  many  lower  animals,  for  examjjle,  the  frog  or 
the  crocodile,  the  growing  tooth  sac  passes  bodily  into  the 


196 


A    MANUAL    OF   DENTAL    ANATOMY. 


excavation  made  before  it  in  the  base  of  the  tooth  which  has 
preceded  it,  while  if  pressure  had  had  any  share  in  the  matter 
the  cells  of  its  enamel  organ,  &c.,  must  have  inevitably  been 
crushed  and  destroyed. 

Again,  when  the  absorption  and  shedding  of  the  first  teeth 

Fig.  85  C). 


have  taken  place  early,  before  their  successors  are  ready  to 
appear,   perfect  little   sockets  are  formed  behind  the  lost 


(^)  Jaws  of  a  six  year  old  child.     In  the  upper  jaw  complete  sockets 
=  een  where  the  temporary  incisors  have  been  shed. 


ERUPTION   OF    THE    TEETH.  197 

temporary  teeth,  cutting  them  off  from  the  permanent  teeth 
destined  to  follow  them.  Absorption,  too,  may  attack  the 
roots  of  permanent  teeth,  which  is  another  reason  for  regard- 
ing the  process  as  not  necessarily  dependent  upon  the  approach 
of  a  displacing  tooth.  Closely  applied  to  the  excavation 
jiroduced  by  absorption  is  a  mass  of  veiy  vascular  soft  tissue, 
the  so-called  absorbent  organ.  The  surface  of  this  is  com- 
posed of  very  large  peculiar-looking  cells,  bearing  some  little 
resemblance  to  those  known  as  "  myeloid  cells,"  or  the 
"  giant  cells  "  of  recent  authors.  Microscopic  examination 
of  the  excavated  surface  shows  it  to  be  covered  with  small 
hemispherical  indentations,  the  "lacunae  of  Howship,"  into 
each  of  which  one  of  the  giant  cells  fitted,  and  in  which  they 
may  sometimes  be  seen  in  situ. 

In  what  manner  these  giant  cells,  or  "  osteoclasts,"  effect 
their  work  is  not  known,  but  their  presence  where  absorption 
of  hard  tissues  is  going  on  is  universal.  Some  suppose  that 
they  put  forth  amoebiform  processes,  others  that  they  secrete 
an  acid  fluid,  but  nothing  very  definite  is  known  ;  a  curious 
parallel  is  afforded  by  the  manner  in  which  a  fungus  can 
di-ill  and  tunnel  through  and  through  the  dentine,  as  may  be 
very  constantly  observed  in  teeth  long  buried. 

The  process  of  absorption  once  commenced  does  not  neces- 
sarily proceed  without  intermission,  but  may  give  place  for 
a  time  to  actual  deposition  of  osseous  tissue  on  the  very  sur- 
face eroded ;  probably  by  the  agency  of  the  absorbent  cells 
themselves,  which  are  capable  of  being  calcified  in  the  exca- 
vations they  have  individually  made. 

These  alternations  of  absorption  and  deposition,  so  com- 
mon a  result  of  inflammations  of  the  pulp,  or  of  the  alveolo- 
dentar  periosteum,  as  to  be  diagnostic  of  the  former  occurrence 
of  these  maladies,  often  occur  during  the  normal  process  of 
the  removal  of  the  deciduous  teeth,  and  result  in  the  deposi- 
tion of.  a  tissue  not  unlike  cementum  in  excavations  made 
in  the  dentine,  or  even  in  the  enamel. 


198  A    MANUAL    OF   DENTAL    ANATOMY. 

The  eruption  of  the  permanent  teeth  is  a  process  closely 
analogous  to  that  of  the  temporary  set.  Rapid  absorption 
of  the  bone,  especially  on  the  exterior  surface  of  the  crypts, 
takes  place,  and  an  orifice  very  much  larger  than  the  crown 
of  the  tooth  is  quickly  opened  out. 

Hence  it  is  that  the  slightest  force  will  suffice  to  determine 
the  direction  assumed  by  the  rising  crown  :  a  fragment  of  a 
root  of  a  temporar}'  tooth,  the  action  of  the  lips  and  tongue, 
&LC.,  are  all  potent  agencies  in  modifying  the  arrangement  of 
the  teeth. 

The  temporaiy  teeth  stood  vertically,  the  })ermanent 
teeth  in  front  of  the  mouth  stand  obliquely,  thus  opening 
a  space  between  the  lateral  incisors  and  the  first  bicuspid 
for  the  canine,  which  during  development  was  out  of  the 
line  altogether.  And,  inasmuch  as  the  crowns  of  the  teeth 
are  on  the  whole  much  larger  than  their  necks,  it  would 
be  manifestly  impossible  for  them  all  to  come  down  simul- 
taneously. 

The  permanent  teeth  usually  make  their  appearance  in 
the  following  order  : — First  permanent  molars,  about  the 
seventh  year ;  a  little  later,  the  lower  central  incisors,  upper 
centrals  and  laterals,  the  first  bicuspids,  the  canines,  the 
second  bicuspids,  the  second  permanent  molars,  the  third 
permanent  molars. 

The  period  of  eruption  is  variable.  From  a  comparison 
of  several  tables,  I  find  the  principal  discrepancies  to  relate 
to  the  date  of  the  appearance  of  the  canines  and  the  second 
bicuspids.  The  canine  would  certainly  appear  to  belong  to 
the  eleventh  and  twelfth  years  ;  but  some  authors  consider 
that  the  second  bicuspid  is  usually  cut  earlier,  otliers  later 
than  this  date. 

We  may  now  revert  to  the  phenomena  observed  in  the 
alveolar  processes.  They  were  first  built  up  as  crypts  with 
overhanging  edges  enclosing  the  temporary  teeth  :  then  they 
were  swept  away,  in  great  part,  to  allow  of  the  eruption  of 


ERUPTION    OF    THE    TEETH.  199 

the  temporary  teeth  :  and  next  they  were  rebuilt  about  the 
necks,  to  form  the  sockets,  of  the  deciduous  teeth. 

Once  more,  at  the  fall  of  the  deciduous  teeth,  the  alveoli 
are  swept  away,  the  crypts  of  the  permanent  teeth  are 
widely  opened,  and  the  permanent  teeth  come  down  through 
the  gaining  orifices. 

When  they  have  done  so,  the  bone  is  reformed  so  as  to 
closely  embrace  their  necks,  and  this  at  a  period  when  but 
little  of  the  root  has  been  completed. 

Take  for  example  the  first  upper  or  lower  molars  :  their 
short  and  widely  open  roots  occupy  the  whole  depth  of  the 
sockets,  and  reach  respectively  to  the  floor  of  the  antrum 
and  the  inferior  dental  canal.  No  growth,  therefore,  can 
possibly  take  place  in  these  directions ;  the  utmost  available 
depth  has  already  been  reached,  and  as  the  roots  lengthen 
the  sockets  must  be  deepened  by  additions  to  their  free 
edges. 

It  is  impossible  to  insist  too  strongly  upon  this  fact,  that 
the  sockets  grow  up  with  and  are  moulded  around  the  teeth 
as  the  latter  elongate.  Teeth  do  not  come  down  and  take 
possession  of  sockets  more  or  less  ready  made  and  pre- 
existent,  but  the  socket  is  subsei'vuent  to  the  position  of  the 
tooth ;  wherever  the  tooth  may  chance  to  get  to,  there  its 
socket  will  be  built  up  round  it. 

Upon  the  proper  appreciation  of  this  fact  depends  our 
whole  understanding  of  the  mechanism  of  teething ;  the 
position  of  the  teeth  determines  that  of  the  sockets,  and  the 
form  of  the  pre-existent  alveolar  bone  has  little  or  nothing 
to  do  with  the  disposition  of  the  teeth. 

During  the  period  of  eruption  of  the  permanent  teeth  the 
level  of  the  alveolar  margin  is  seen,  in  a  dried  skull,  to  be 
extremely  irregular,  the  edges  of  the  sockets  corresponding  to 
tlie  necks  of  the  teeth,  whether  they  have  attained  to  their 
ultimate  level,  or  have  been  but  just  cut. 

And  when  temporary  teeth  have  been  retained  for  a  longer 


200 


A    MANUAL    OF   DENTAL    ANATOMY. 


period  than  is  natural,  they  sometimes  become  elevated  to 
the  general  level  of  the  permanent  teeth  (which  is  consider- 
ably higlier  than  that  of  the  temporary  teeth),  so  that  they 
take  their  share  of  work  in  mastication.  When  this  is  the 
case  the  alveoli  are  developed  round  them,  and  come  to 
occupy  with  the  tooth  a  higher  level  than  before. 


Fig.  86  ('). 


Enough  has  perhaps  been  said  to  illustrate  the  entire 
dependence  of  the  alveoli  upon  the  teeth,  a  relation  of  which 
dentists  every  day  avail  themselves  in  the  treatment  of 
regulation  cases  :  it  remains  to  say  a  few  words  as  to  the 
forces  which  do  determine  the  position  of  the  teeth. 

Inasmuch  as  when  a  tooth  leaves  its  bony  crypt,  the 
bone  does  not  at  first  closely  embrace  it,  but  its  socket  is 

{})  From  a  child  aged  fourteen.  The  specimen  well  exemplifies  the  fact 
that  the  height  of  the  alveolar  edge  corresponds  exactly  to  the  position  of 
the  neck  of  each  tooth,  on  which  it  is  wholly  dependent.  A  temporary 
tooth  (the  first  right  lower  temporary  molar)  has  been  elevated,  so  that  it 
has  attained  to  the  level  of  the  surrounding  permanent  teeth,  and  the  edge 
of  the  socket  follows  the  level  of  the  neck  of  the  tooth. 


ERUPTION   OF    THE    TEETH.  201 


miich  too  large  for  it,  a  very  small  force  is  sufficient  to 
deflect  it.  And,  indeed,  a  very  slight  force,  constantly 
operating,  is  sufficient  to  materially  alter  the  position  of  a 
tooth,  even  when  it  has  attained  to  its  full  length. 

Along  the  outside  of  the  alveolar  arch  the  muscular  lips 
are  exercising  a  very  symmetrical  and  even  pressure  upon 
the  crowns  of  the  teeth ;  so  also  the  tongue  is,  with  equal 
symmetry,  pushing  them  outwards :  between  the  two  forces, 
the  lips  and  the  tongue,  the  teeth  naturally  become  moulded 
into  a  symmetrical  arch.  That  the  lips  and  tongue  are  the 
agencies  which  mainly  model  the  arch  is  very  well  illustrated 
by  that  which  happens  in  persons  who  have  from  childhood 
suffered  from  enlargement  of  the  tonsils,  and  are  conse- 
quently obliged  to  breathe  through  the  mouth,  which  is  thus 
pretty  constantly  open.  This  causes  a  slight  increase  in  the 
tension  of  the  lips  at  the  corners  of  the  mouth,  and  is  im- 
pressed upon  the  alveolar  arch  as  an  inward  bending  of  the 
bicuspids  at  that  point ;  thus  persons  with  enlarged  tonsils 
will  be  found,  almost  invariably,  to  present  one  of  the  forms 
of  mouth  known  as  V-shaped. 

But  Dr.  Norman  Kingsley  attaches  far  more  importance 
to  disturbed  innervation  than  to  any  mechanical  causes,  and 
refers  most  dental  iiTegularities  to  unhealthy  conditions  of 
the  child's  nervous  system. 

When  the  crowns  of  the  teeth  have  attained  such  a  level 
as  to  come  in  contact  with  their  opposing  teeth,  they  very 
speedily,  from  readily  intelligible  mechanical  causes,  are 
forced  into  a  position  of  perfect  correspondence  and  an- 
tagonism ;  and  even  at  a  somewhat  later  period  than  that 
of  eruption,  if  this  antagonism  be  interfered  with,  the  teeth 
will  often  rise  up  so  as  to  readjust  themselves  in  position. 


202  A    MANUAL    OF   DENTAL    ANATOMY. 


THE    ATTACHiMENT    OF    TEETH. 

Although  the  various  methods  by  which  teeth  are  fixed  iu 
their  position  upon  the  bones  which  carrj^  them  pass  by 
gradational  forms  into  one  another,  so  that  a  simple  and  at 
the  same  time  absolutely  correct  classification  is  impossible, 
yet  for  the  purposes  of  description  four  principal  methods 
may  be  enumerated,  namely,  attachment  by  means  of  fibrous 
membrane,  by  a  hinge,  by  anchylosis,  and  by  implantation 
in  bony  sockets. 

Attachment  toy  means  of  Fibrous  Membrane. — An 
excellent  illustration  of  this  manner  of  implantation  is 
afforded  by  the  Sharks  and  Rays,  in  which  the  teeth  have 
no  dii'ect  connection  with  the  cartilaginous,  more  or  less 
calcified,  jaws,  but  are  imbedded  solely  in  the  tough  fibrous 
mucous  membrane  which  covers  them.  This,  carrying  with 
it  the  teeth,  makes  a  sort  of  sliding  progress  over  the  curved 
surface  of  the  jaw,  so  that  the  teeth  once  situated  at  the 
inner  and  lower  border  of  the  jaw,  where  fresh  ones  are 
constantly  being  developed,  rotate  over  it,  and  come  to 
occupy  the  topmost  position  (cf.  description  of  the  denti- 
tion of  the  sharks).  That  the  whole  fibrous  giim,  with  the 
attached  teeth,  does  really  so  slide  over  the  surface  of  the 
jaw,  was  accidentally  demonstrated  by  the  result  of  an  injury, 
which  had  been  inflicted  upon  the  jaws  of  a  shark. 

The  fibrous  bands  by  which  each  individual  tooth  of  the 
shark  is  bound  down  are  merely  portions  of  that  same  sheet 
of  mucous  membrane  which  furnished  the  dentine  papillte ; 
and  the  gradual  assumption  of  the  fibrillated  structure  by 
that  portion  of  the  mucous  membrane  which  is  contiguous 
to  the  base  of  the  dentine  papilla  may  be  traced,  no  such 
fibrous  tissue  being  found  at  the  base  of  young  papillae,  and 
very  dense  bands  being  attached  to  the  bases  of  the  com- 
pleted calcified  teeth. 

Attachment   by  an  Elastic    Hinge. — The  possession 


THE   ATTACHMENT    OF    TEETH.  203 

of  moveable  teeth,  able  to  yield  to  pressure  and  subsequently 
to  resume  the  upright  position,  was  formerly  supposed  to  be 
confined  to  the  Lophius  (Angler)  and  its  immediate  allies. 
I  have  however  found  hinges  in  the  common  Pike  (Esox), 
and  in  the  Gadidce  (Cod  tribe) ;  so  that,  as  they  occiu'  in  these 
fish  so  widely  removed  from  one  another  in  other  respects,  it  is 
probable  that  further  investigation  will  bring  to  light  many 
other  examples  of  this  very  peculiar  method  of  attachment, 
eminently  suited  to,  and  hitherto  only  discovered  in,  fish  of 
predatory  habits. 

In  the  Angler,  which  obtains  its  food  by  lying  in  ambush 
on  the  bottom,  to  which  it  is  closely  assimilated  in  colour, 
many  of  the  largest  teeth  are  so  hinged  that  they  readily 
allow  an  object  to  pass  into  the  mouth,  but  rebounding 
again,  oppose  its  egress.  These  teeth  are  held  in  position 
by  dense  fibrous  ligaments  radiating  from  the  posterior  side 
of  their  bases  on  to  the  subjacent  bone,  while  the  fronts  of  the 
bases  of  the  teeth  are  free,  and  when  the  teeth  are  pressed 
towards  the  throat,  rise  from  the  bone.  The  elasticity  of 
the  ligament  is  such  that  when  it  has  been  compressed  by 
the  tooth  being  over  towards  it,  it  returns  it  instantly  into 
position  with  a  snap.  Many  of  the  teeth  of  the  Angler  are, 
like  most  fishes'  teeth,  anchylosed  firmly. 

The  Hake  (Merlucius,  one  of  the  Gadidce)  possesses  two 
rows  of  teeth,  the  inner  and  shorter  of  which  are  anchylosed, 
whilst  the  outer  and  longer  are  hinged. 

In  some  respects  these  hinged  teeth  are  more  highly 
specialised  than  those  of  the  Angler,  which  they  resemble 
in  being  attached  by  an  elastic  hinge  fixed  to  their  inner 
sides,  the  elasticity  of  which  is  brought  into  play  by  its  being 
compressed,  or  at  all  events  bent  over,  upon  itself. 

The  pulp  is  highly  vascular,  and  its  vessels  are  so  arranged 
that,  by  entering  the  pulp  through  a  hole  in  the  ligament, 
which  is  about  at  the  axisof  motion,  they  escape  being  stretched 
or  torn  during  the  movements  of  the  tooth.     But  the  base 


204  A    MANUAL    OF  DENTAL    ANATOMY. 

of  the  tooth  itself  is  modified  so  as  to  be  pai'ticularly  fitted 
for  resisting  the  jars  to  which  a  moveable  tooth  must  at  times 
be  exposed,  and  so  is  the  bone  upon  which  it  is  set. 

As  is  seen  in  the  figure,  the  base  of  the  tooth,  or  the  side 

Fio.  87  (')• 


opposite  to  the  hinge,  is  thickened  and  rounded,  the  advan- 
tages which  such  a  form  must  possess  over  a  thin  edge  when 
bumping  upon  the  bone  being  sufiiciently  obvious.     This 

(')  Hinged  tooth  of  Hake.  a.  Vaso  dentine,  h.  Pulp.  c.  Elastic 
hinge,  d.  Buttress  of  bone  to  receive  /,  formed  out  of  bone  of  attach- 
ment.    €.  Bone  of  jaw.     f.  Thickened  base  of  tooth,     g.   Enamel  tip. 


THE   ATTACHMENT    OF    TEETH.  205 

thickened  edge  is  received  upon  a  little  buttress  of  bone, 
and  it  occupies  a  much  higher  level  than  the  opposite  thin 
edge  to  which  the  hinge  is  attached,  so  that  the  tooth  cannot 
possibly  be  bent  outwards  without  actual  rupture  of  the 
ligament. 

And  what  is  not  a  little  remarkable  is,  that  whilst  the 
Hake,  the  most  predatory  of  all  the  Gadidce,  is  possessed  of 
these  very  perfectly  hinged  teeth,  other  members  of  the 
family  have  teeth  moveable  in  a  less  degree,  whilst  others 
again  have  teeth  rigidly  fixed.  So  that  within  the  limits  of 
a  single  family  we  have  several  steps  in  a  gradual  progres- 
sion towards  a  very  highly  specialised  organ. 

In  the  hinged  teeth  already  alluded  to  the  purpose  served 
by  their  mobility  seems  to  be  the  catching  of  active  fish,  and 
the  elasticity  resides  solely  in  the  hinges  ;  but  the  common 
Pike  possesses  many  hinged  teeth  which  seem  to  be  con- 
cerned in  the  swallowing  of  the  prey  after  it  has  been  caught, 
and  there  is  no  elasticity  in  the  hinges,  the  resilience  of  the 
teeth  being  provided  for  in  another  way. 

The  teeth  which  surround  the  margins  of  the  jaws  are 
anchylosed,  and  they  are  more  or  less  solidly  filled  up  in 
their  interior  with  a  development  of  osteodentine,  which,  by 
becoming  continuous  with  the  subjacent  bone,  cements  them 
upon  it.  The  manner  of  development  of  this  is  by  rods  of 
calcifying  material  shooting  down  through  the  central  pulp 
(see  page  16.5) ;  in  the  hinged  teeth  also  these  trabeculse  shoot 
down,  and  become  continuous  with  the  subjacent  bone,  only 
instead  of  rigidly  ossifying  they  remain  soft  and  elastic,  so 
that  the  tooth  is  like  an  extinguisher  fastened  down  by  a 
large  number  of  elastic  strings  attached  to  different  points 
on  its  interior,  and  hinged  at  one  side. 

The  elasticity  is  very  perfect,  so  that  the  teeth  depressed 
and  suddenly  released  return  with  an  audible  snap,  but  it 
resides  solely  in  these  strings,  for  if  these  be  divided  by 
carefully  slipping  a  cataract  needle  under  the  tooth  without 


206  A    MANUAL    OF   DENTAL    ANATOMY. 

injuring  the  binge,  the  tooth  will  stay  in  any  position  into 

which  it  is  put. 

Fio.  88  ('). 


The  points  most  noteworthy  are,  (i.)  that  hinged  teeth  have 
arisen  independently  in  three  families  of  fish  widely  removed 
from  one  another,  and  (ii.)  that,  whilst  the  general  object  of 
mobility  and  elastic  resiliency  is  attained  in  all,  it  is  by  a 
different  mechanism,  and  by  the  least  modification  possible 
of  the  existing  fixed  teeth  of  the  family. 

(1)  Hinged  tooth  of  Pike.  a.  Dentine,  h.  Elastic  rods,  formed  of  un- 
calcified  trabeculoe  which  might  have  become  bone.  c.  Hinge,  not  itself 
elastic,     d.  Bone  of  attachment,     c.  Bone  of  body  of  jaw. 


THE   ATTACHMENT    OF    TEETH.  207 


Attachment  by  Anchylosis. — In  both  the  socketed  and 
the  membranous  manners  of  attachment  an  organised,  more 
or  less  vascular  membrane,  intervenes  between  the  tooth  and 
the  jaw-bone  ;  in  the  method  now  under  consideration  there 
is  no  such  intervening  membrane,  but  the  calcified  tooth 
substance  and  the  bone  are  in  actual  continuity,  so  that  it 
is  often  difficult  to  discern  with  the  naked  eye  the  line  of 
junction. 

The  teeth  may  be  only  slightly  held,  so  that  they  break 
oflf  under  the  application  of  only  a  moderate  degree  of  force, 
or  they  may  be  so  intimately  bound  to  the  bone  that  a 
portion  of  the  latter  will  usually  be  torn  away  with  the 
tooth. 

A  very  perfect  example  of  attachment  by  anchylosis  is 
afforded  by  the  fixed  teeth  of  the  Pike,  of  which  the  central 
cone  is  composed  of  osteodentine.  The  method  by  which  the 
entire  fusion  of  this  tissue  with  the  bone  beneath  it  takes 
place  has  ah'eady  been  alluded  to,  the  similarity  of  its 
method  of  calcification  with  that  of  bone  rendering  the 
fusion  easy  and  complete. 

And  in  certain  extinct  fish,  whose  nearest  ally  is  the  now 
anomalous  Australian  shark,  the  Cestracion  philippi,  the 
lower  part  of  the  tooth  is  composed  of  osteodentine,  which 
so  closely  resembles  bone  itself  that  it  is  impossible  to  say 
at  which  point  the  bone  may  be  said  to  commence  and  the 
tooth  to  end  ;  but  even  where  this  intimate  resemblance  in 
histological  character  does  not  exist,  there  is  often  to  be 
found  more  or  less  blending  of  the  basal  dentine  with  the 
bone  beneath  it,  so  that  there  is  even  here  a  sort  of  transi- 
tional region. 

From  the  accounts  which  pass  current  in  most  text  books 
it  would  be  supposed  that  the  process  of  attachment  by 
anchylosis  is  a  vezy  simple  matter,  the  base  of  the  dentine 
papilla,  or  the  dental  capsule,  by  its  calcification  cementing 
the  tooth  on  to  a  surface  of  the  jaw-bone  already  formed. 


208  A    MANUAL    OF   DENTAL    ANATOMY. 

In  the  few  animals  which  I  have  examined  ('),  however,  I 
have  found  that  this  conception  does  not  at  all  adequately 
represent  what  really  takes  place  ;  it  seldom,  perhaps  never, 
happens  that  a  tooth  is  attached  directly  to  a  plane  surface 
of  the  jaw  which  has  been  formed  previously  ;  but  the  union 
takes  j)lace  through  the  medium  of  a  portion  of  bone  (which 
may  be  large  or  small  in  amount)  which  is  specially  developed 


to  give  attachment  to  that  one  particular  tooth,  and  after 
the  fall  of  that  tooth  is  itself  removed. 

For  this  bone  I  have  proposed  the  name  of  "bone  of 
attachment,"  and  it  is  strictly  analogous  to  the  sockets  of 
those  teeth  which  have  sockets.  It  is  well  exemplified  in 
the   Ophidia,  a  description  of  the  fixation  of  the  teeth  of 

Q)  Transactions  of  the  Odontological  Society,  Dec.  1874. — "Studies  on 
the  Attachment  of  Teeth." 

(-)  Section  of  tooth  and  a  portion  of  the  jaw  of  a  Pytlion,  showing  the 
marked  difference  in  character  between  the  bone  of  attachment  and  the 
rest  of  the  bone. 


THE   ATTACHMENT    OF    TEETH.  209 


which  will  serve  to  convey  a  good  idea  of  its  geucral  cha- 
racter. If  the  base  of  one  of  the  teeth,  with  the  subjacent 
jaw-bone,  be  submitted  to  microscopic  examination  we  shall 
find  that  the  layer  of  bone  which  closely  embraces  the  tooth 
contrasts  markedly  with  the  rest  of  the  bone.  The  latter  is 
fine  in  texture,  its  lacunce,  with  their  very  numerous  fine 
canaliculi,  very  regular,  and  the  lamination  obviously  refer- 
able to  the  general  surface  of  the  bone.  But  the  "  bone 
of  attachment "  is  very  coarse  in  texture,  full  of  irregular 
spaces,  very  different  from  the  regular  lacunae,  and  its  lamin- 
ation is  roughly  parallel  with  the  base  of  the  tooth.  The 
dentine  of  the  base  of  the  tooth  also  bends  inwards  (Fig.  89), 
and  its  tubes  are  lost  in  the  osseous  tissue,  a  blending  so 
intimate  resulting,  that  in  grinding  down  sections  the  tooth 
and  the  bone  of  attachment  often  come  away  together,  the 
tooth  and  this  bone  being  more  intimately  united  than  this 
special  bone  is  with  that  of  the  rest  of  the  jaw. 

A  study  of  its  development  also  proves  that  it  has  an  in- 
timate relation  with  the  tooth  with  which  it  is  continuous, 
for  it  is  wholly  removed  with  the  Ml  of  the  tooth,  and  is 
specially  developed  again  for  the  next  tooth  which  comes 
into  position.  The  periosteum  of  the  rest  of  the  jaw-bone 
appears  to  take  an  important  share  in  the  formation  of  this 
special  bone  substance,  and  the  tooth  capsule,  by  its  ossifi- 
cation, apparently  contributes  little. 

In  the  frog  the  teeth  are  commonly  described  as  being- 
attached  by  their  bases  and  outer  surfiicc  to  a  continuous 
groove,  of  which  the  external  wall  is  the  highest.  Such  is, 
however,  an  inadeqiiate  description  of  the  process,  the 
tooth,  as  seen  in  section,  being  attached  on  its  outer  side 
by  a  new  development  of  special  bone,  which  extends  for  a 
short  distance  up  over  its  external  surface  ;  and  for  the 
support  of  its  inner  wall  there  springs  up  from  the  sub- 
jacent bone  a  pillar  of  bone,  which  is  entirely  removed  when 
that  tooth  falls,  a  new  pillar  being  developed  for  the  next  tooth. 


210  A    MANUAL    OF   DENTAL    ANATOMY. 

When  the  teetli  arc,  as  in  many  fish,  implanted  upon 
what  to  the  naked  eye  appears  nothing  more  than  a  plane 
surface  of  bono,  a  microscopic  examination  generally,  in  fact 
in  all  specimens  which  I  have  examined,  reveals  that  the 
individual  teeth  are  implanted  in  depressions  much  larger 
than  themselves,  the  excess  of  space  being  occupied  by  new 
and  specially  formed  bone,  or  else  that  the  teeth  surmount 
pedicles,  which  are  closely  set  together,  the  interspaces  being 
occupied  with  a  less  regular  calcified  structure. 

A  good  example  of  the  latter  method  is  afforded  by  the  Eel 

Fig.  90  ('). 


(Fig.  90),  in  which  each  tooth  surmounts  a  short  hollow  cylin- 
der of  bone,  the  lamination,  &c.,  of  which  differs  strongly 
from  that  of  the  body  of  the  jaw-bone.  When  the  tooth  which 
it  carries  is  shed,  the  bone  of  attachment,  in  this  case  a  hollow 
cylinder,  is  removed  right  down  to  the  level  of  the  main 
bone  of  the  jaw,  as  is  well  seen  in  the  figure  to  the  left  of 
the  teeth  in  position.      Under  a  higher  magiiifying  power 

(')  From  lower  jaw  of  an  Eel.     a.  Bone  of  jaw.     h.   Bone  of  attach- 
ment,    d.  Dentine.     /.   Enamel,     y.   Space  vacated  by  a  shed  tooth. 


THE   ATTACHMENT    OF    TEETH. 


the  bone  at  this  point  would  be  found  to  be  excavated  by 
"  How'ship's  lacunae."  As  an  anchylosis,  the  implantation 
of  the  teeth  is  less  perfect  than  that  of  those  of  the  snake, 
for  the  dentinal  tubes  at  the  base  of  the  tooth  are  not  de- 


flected, and  do  not  in  any  sense  blend  witli  the  bone  beneath 
them.  Accoi'dingly,  the  teeth  are  fir  loss  firmly  attached, 
and  bi-eak  off  quite  readily. 

A  transition  towards  the  socketed  type  of  implantation  is 
furnished  by  some  of  the  cod  tribe.  In  the  haddock,  for 
example,  the  teeth  surmount  hollow  cylinders  of  "  bone  of 
attachment,"  resembling  in  many  particulars  those  of  the 
eel;  the  teeth  do  not,  however,  simply  surmoimt  the  bonv 
cylinders,  but  are  continued  for  a  short  distance  within 
them,  definite  shoulders  being  formed  which  rest  on  the 
rims  of  the  cjdinder.  The  base  of  the  tooth  does  not,  how- 
ever, contract  or  taper  any  more,  and  is  widely  open,  so  that 
it  cannot  be  considered  that  any  close  approximation  to  a 
root  is  made.     The  pulp  cavity  of  the  tooth  becomes  con- 


(^)  From  lower  jaw  of  a  Haddock, 
inent,     rf.  Dentine  of  tootJi. 


a.  Bone  of  jaw.     /;.  lione  of  attach- 


212  A    MANUAL    OF   DENTAL    ANATOMY. 


tinuous  with  the  cavity  of  the  osseous  cylinder,  into  which 
it  is  for  a  shoi't  distance  continued. 

The  bony  supports  of  the  teeth  originate  in  many  osseous 
trabecula)  whicli  spring  up  simultaneously  from  the  lione  of 
the  jaw  beneath  the  new  tooth  ;  these  coalesce  to  form  a 
net-like  skeleton,  which  rapidly  becomes  filled  in  by  the 
progress  of  ossification.  So  far  as  my  own  researches  enable 
me  to  say,  there  is  this  much  in  common  in  all  forms  of 
attachment  by  anchylosis,  no  matter  how  different  the- 
naked  eye  results  of  the  process  may  be ;  the  tooth,  as  it 
comes  into  position,  is  secured  by  an  exceedingly  rapid 
development  of  bone,  which  is  more  or  less  directly  an 
outgrowth  from  the  jaw-bone  itself,  which  is  in  some  im- 
seen  manner  stimulated  into  activity  by  the  proximity  of  the 
tooth.  In  amount  this  specially  formed  bone  varies  greatly^ 
but  in  all  instances  it  is  not  the  tooth  capsule,  but  tissue* 
altogether  external  to  this,  which  serve  to  secure  the  tooth 
in  its  place  by  their  ossification. 

The  teeth  of  the  maclcerel  present  an  interesting  variety  of 
attachment  by  anchylosis.  The  margins  of  the  jaws  are  very 
thin,  and  by  no  means  fleshy,  and  in  this  thin  margin  there 
is  a  deep  groove  between  the  outer  and  inner  plate  of  the 
bone.  In  this  groove  are  the  teeth,  their  sharp  points  pro- 
jecting beyond  the  edges  of  the  bone,  and  they  are  held  in 
their  place  by  a  network  or  scaffolding  of  bone  of  attach- 
ment which  is  developed  between  their  sides  and  the  inner 
surface  of  the  bone.  They  are,  so  to  speak,  hung  up  in  their 
place,  and  their  open  bases  rest  on  nothing,  or  at  least  on 
nothing  hard. 

Attachment  by  implantation  in  a  socket. — In  this, 
as  in  anchylosis,  there  is  a  special  development  of  bone, 
which  is  modelled  to  the  base  of  the  tooth,  but  instead  of 
its  being  in  actual  close  continuity  with  the  dental  tissues, 
there  intervenes  a  vascular  organised  membrane.  The 
manner  in  which  the  sockets  are,  so  to  speak,  plastered 


THE   ATTACHMENT    OF    TEETH.  213 

around  the  roots  of  the  teeth,  and  are  perfectly  subservient 
to  and  dependent  on  them,  has  already  been  described ;  little, 
therefoi-e,  need  be  added  here,  save  that  the  soft  tissue 
intervening  between  the  bone  and  the  tooth  is  not  sepa- 
rable, either  anatomically  or  from  the  point  of  view  of 
-development,  into  any  two  layers,  but  is  a  single  mem- 
brane, termed  the  "aiveolo-den tar-periosteum."  That  it  is 
single,  is  a  matter  of  absolute  certainty ;  there  is  no  difficulty 
in  demonstrating  it  in  situ,  with  vessels  and  bundles  of 
fibres  traversing  its  whole  thickness  from  the  tooth  to  the 
bone,  or  vice  versa. 

The  nature  and  develojiment  of  the  sockets  in  those 
few  reptiles  and  fishes  whicli  have  socketed  teeth  require 
further  examination.  I  am  not,  from  what  I  have  seen  in 
sections  of  the  jaws  of  a  young  crocodile,  inclined  to  regard 
them  as  in  all  respects  similar  to  the  alveoli  of  mammalian 
teeth.  At  all  events  they  are  not  developed  in  that  same 
subsex'viency  to  each  individual  tooth ;  on  the  contrary, 
successive  teeth  come  up  and  occupy  a  socket  which  is 
•already  in  existence. 

Although  there  are  animals  in  which  implantation  in  a 
spurious  socket  is  supplemented  by  anchylosis  to  the  wall 
or  to  the  bottom  of  the  socket,  no  example  of  anchylosis 
occm-ring  between  the  tooth  and  the  bone  of  the  socket  has 
€ver  been  met  with  in  man,  or  indeed  in  any  mammal  ex- 
•emplifyiug  a  typical  socketed  implantation  of  the  teeth. 

Hunter.     On  the  Anatomy  of  the  Human  Teeth. 

Tomes,  J.     Dental  Surgery.     1859. 

HUMPHERY.     Ti-ansact.  Camb.  Philos.  Soc.     1863. 

Wedl.    Pathology  of  the  Teeth. 

Heudker.      Beitrage  zur   Lehre  von  der    Knochenentwickelung, 
&c. 

Tomes,  Charles  S.     On  Vascular   Dentine  and  Hinged   Teeth. 
Philos.  Transac,  1878,  and  Quart.  Journal 
Micros.  Science,  vol.  xvii.  new  series. 
Ti-ansac.  Odontolog.  Soc.     1874 — 187G. 


CHAPTER  VI. 

THE    TEETH    OF   FISHES. 

In  the  following  pages  nothing  more  than  a  brief  account 
of  a  few  typical  forms  can  be  attempted  ;  the  limits  of  space 
forbid  the  mention  of  many  creatures,  or  the  insertion  of 
detailed  descriptions  of  the  dentition  even  of  the  few  which 
are  included  in  its  pages.  In  the  class  of  fish  the  task  of 
selection  of  the  forms  for  description  is  no  easy  one ;  for 
the  almost  infinite  diversity  of  dentition  which  exists  in 
it  makes  it  a  matter  of  peculiar  difficulty  to  frame  any 
general  account,  or  to  do  more  than  present  before  the 
reader  a  description  of  a  few  individual  forms  from  which  he 
may  gather,  as  best  he  can,  a  general  idea  of  piscine 
dentition. 

Fish  may  be  grouped  into — 

I.  Phabyngobranchii.  IV.  Gaxoidei, 

II.  Marsipobranchii.  v.  Teleostei. 

III.  Elasmobbanchii.  VI.  Dipnoi. 

I.  PlinrymjobrancMl  comprise  only  the  Amphio.inis. 
II.  MarxipobrancMi   comprise   the   Lampreys    and    the    parasitic 
Myxine. 

III.  Elii.sinohranc?i,ii  comprise  the  Sharks  and   Eays  (Phir/iostovii) 

and  the  Chimera  and  its  allies  {IMocapJiali).    Their  skele- 
tons are  cartilaginous,  with  an  ossified  crust. 

IV.  (rtuinidri.    A  large  number  of  extinct  fish — of  existing  fish  the 

Lrpithititm.^.  or  Bony  Pike,  is  the  most  familiar. 
V.   Tclcoxtei  comprise  the  ordinary  Fish  in  our  seas  and  rivers. 
VI.  B'lpmn.    The  LepUlosircnK,  or  Mud  Fishes,  capable  of  living  for 
a  long  time  in  moist  mud. 

The  Marsipohrancldi  need  not  detain  us  long ;  they  are 


THE    TEETH    OF   FISHES.  215 

destitute  of  true  calcified  or  dentinal  teeth,  the  armature  of 
the  mouth  consisting  of  horny  cones  or  serrated  plates. 

The  parasitic  Myxine,  which  is  found  in  the  interior  of 
other  larger  fish,  is  furnished  with  a  median  curved  conical 
tooth,  of  horny  consistency,  which  is  believed  to  act  as  a 
holdfast,  while  the  serrated  edges  of  the  horny  plates  upon 
the  tongue  are  brought  into  play  in  boring  a  way  into  the 
interior  of  its  host. 

The  concave  circular  disc  which  suiTounds  the  mouth  of 
the  Lamprey  is  covered  with  concentrically  disposed  horny 
teeth,  of  simple  conical  form  ;  in  addition  to  these  there 
are  lingual  and  palatal  horny  plates. 

The  teeth  of  Elasmohranchii  present  rather  more  of  sim- 
plicity and  uniformity  of  plan  than  do  those  of  most  fish, 
and  it  will  hence  be  convenient  to  describe  their  teeth  first, 
although  ill  most  respects  they  stand  at  the  head  of  the 
class  of  fishes,  and  present  many  indications  of  affinity  with 
the  Batrachia. 

In  the  Plagiostomi  the  mouth  is  a  transverse,  more  or  less 
curved  fissure,  opening  upon  the  under  surface  of  the  head 
at  some  little  distance  behind  the  end  of  the  snovit.  Hence 
it  is  that  a  shark  in  seizing  its  prey  turns  over  upon  its  back 
or  at  all  events  upon  its  side. 

The  jaws,  which  are  made  up  of  the  representatives  of  the 
palato-quadrate  arch,  and  of  Meckel's  cartilage,  neither  true 
maxillae  nor  premaxillse  being  present,  are  cartilaginous  in 
the  main  (although  covered  with  a  more  or  less  ossified  crust), 
and  therefore  shrink  and  become  much  distorted  in  drying. 
The  shape  of  the  jaws  differs  in  the  various  groups  of 
Playiostomi,  in  some  each  of  the  two  jaws  being  a  tolerably 
perfect  semicircle,  while  in  others  they  are  nearly  straight 
and  pai'allel  to  one  another  (see  fig.  92  and  fig.  96) ;  but  in  all 
the  rounded  working  surface  of  the  jaw  is  clothed  or  encased 
by  teeth,  which  are  arranged  in  many  parallel  concentric 
rows. 


216 


A    MANUAL    OF   DENTAL    ANATOMY. 


The  teeth,  which  are  situated  upon  the  edge  or  exposed 
border  of  the  jaw,  arc  usually  erect,  whilst  the  rows  which 
lie  behind  them,  farther  within  the  mouth,  point  backwards, 
and  are  more  or  less  recumbent,  not  having  yet  come  into 
full  use. 

In  this  respect,  however,  marked  difference  exists  among 
various  genera  of  sharks ;  for  instance  in  the  great  trojjical 

Fig.  92  (';. 


t  n.  s. 


white  shark  the  teeth  which  lie  on  the  border  of  the  jaw 
are  erect,  and  all  the  successive  rows  are  quite  recumbent, 
whereas  in  many  of  the  dog-fishes  the  inner  surfice  of  the 
jaws  forms  an  even  rounded  surface  along  which  the  rows  of 
teeth  are  disposed  in  every  intermediate  position  between 
those  fully  recumbent  at  the  innermost  pai-t  of  the  jaw, 
and  those  fully  erected  upon  its  exposed  borders.  Only  a 
few  of  the  most  forward  rows  of  teeth  are  exposed,  a  fold 
or  flap  of  mucous  membrane  covering  in  those  teeth  which 


(^)  Lower  jaw  of  Lamna.     a.  Edge  of  flap  of  mucous  membrane  wliich 
covers  in  the  teeth  not  yet  completed. 


THE    TEETH    OF    FISHES. 


217 


are  not   as  yet   fully  calcified  and  firmly   attached  to   the 
gum. 

In  Lamna,  which  may  be  taken  as  fairly  illustrative,  the 
teeth  are  arranged  round  the  jaws  in  concentric  rows  with 
gi-eat  regularity,  the  teeth  of  the  successive  rows  con-espond- 
ing  in  position  to  the  teeth  of  older  rows,  and  not,  as  is  the 
case  in  some  other  sharks,  to  their  interspaces.     They  are 


attached  by  being  embedded  in  a  densely  fibrous  gum,  which 
closely  embraces  their  bifurcated  bases  ;  and  this  dense  gum, 
carrying  with  it  the  teeth,  slides  bodily  upwards  over  the 
inner  face  of  the  jaw,  and  outwards  over  its  border,  beyond 


(')  Transverse  section  of  lower  jaw  of  a  Dog-fish.  «.  Oral  epithelium. 
i.  Oral  epithelium  passing  on  to  flap.  c.  Protecting  flap  of  mucous  mem- 
brane (thecal  fold),  d.  Youngest  dentine  pulp.  e.  Youngest  enamel 
organ.     /.   Tooth  about  to  be  shed.     g.  Calcified  crust  of  jaw. 


218  A    MANUAL    OF   DENTAL   ANATOMY. 


which  it,  to  borrow  a  phrase  from  geological  science,  has  an 
"  outcrop." 

In  Lamna  the  second  and  third  rows  of  teeth  arc  only 
partially  erect,  the  rows  behind  these  lying  recumbent,  and 
being  in  the  fresh  state  covered  in  V)y  the  fold  of  mucous 
membrane,  which,  being  dried  and  shrunk  in  the  specimen 
figured,  falls  short  of  its  original  level. 

Thus  rows  of  teeth  originally  developed  at  the  base  of 
the  jaw  arc  carried  upwards,  come  to  occupy  the  foremost 
position  on  the  border  of  the  jaw,  and  are  cast  off  when 
they  pass  the  point/  in  the  figure.  It  is  thus  easy  to  under- 
stand why  sharks'  teeth  are  so  abundantly  found  in  a  fossil 
condition,  although  other  indications  of  the  existence  of  the 
fish  are  rare  enough  ;  for  every  shark  in  the  course  of  its 
life  casts  off  great  numbers  of  teeth,  which  fall  to  the 
bottom  of  the  sea  and  become  bedded  in  the  deposit  there 
forming. 

The  teeth  are  never  anchylosed  to  the  jaw,  nor  have  they 
any  direct  connection  with  it,  but,  as  before  mentioned,  are 
retained  by  being  bedded  in  a  very  tough  fibrous  mem- 
brane ;  the  natiu-e  of  then-  fixation  has  been  more  exactly 
described  at  another  page  (page  202). 

The  sheet  of  fibrous  gum  slides  bodily  over  the  curved 
surface  of  the  jaw,  continually  bringing  up  from  below 
fresh  rows  of  teeth,  as  was  proved  by  Andre's  specimen, 
and  it  may  be  worth  while  to  condense  from  Professor 
Owen  the  description  of  the  manner  in  which  it  was  thus 
proved  that  an  actual  sliding  or  rotation  of  the  membrane 
does  really  take  place,  and  that  the  whole  bony  jaw  itself 
does  not  become  slowly  everted.  The  spine  of  a  sting  ray 
had  been  driven  through  the  lower  jaw  of  a  shark  (Galeus), 
passing  between  two  (vertical)  rows  of  teeth  which  had 
not  yet  been  brought  into  use  ;  when  the  specimen  caine 
under  observation  the  spine  had  remained  in  this  situation, 
transfixing  the  jaw,  for  a  long  time,  as  was  evidenced  by  all 


THE    TEETH    OF   FISHES.  219 

the  teeth  of  these  two  rows,  both  above  and  below  it,  being 
stunted  aad  smaller  than  their  neighbours. 

Hence  the  development  of  these  teeth,  which  ultimately 
came  to  be  at  some  little  distance  from  the  spine,  had  been 
profoundly  modified  by  its  presence,  and  it  is  difficult  to 
understand  in  what  manner  this  could  have  affected  them 
had  they  not,  at  an  earlier  period  of  their  growth,  lain  in 
more  immediate  proximity  to  it.  But  if  the  membrane, 
with  the  teeth  attached,  does  move  slowly  along  the  surface 
of  the  jaw,  this  difficulty  at  once  disappears. 

The  forms  of  the  teeth  in  various  sharks  are  different 
and  characteristic ;  nevertheless  they  vaiy  somewhat  with 
age  in  some  species,  and  present  differences  in  size  and 
form  in  the  upper  and  lower  jaws,  or  in  different  parts  of  the 
mouth  of  the  same  individual.  For  instance,  in  Lamna,  in 
the  upper  jaw,  the  third  teeth  of  each  horizontal  row,  count- 
ing from  the  middle  line,  are  very  small,  while  in  both  jaws 
there  is  a  gradual  diminution  in  the  size  of  the  teeth  towards 
the  back  of  the  mouth. 

Thus,  although  it  is  often  possible  to  refer  a  particular 
tooth  to  its  right  genus  or  even  species,  much  care  is  re- 
quisite in  so  doing. 

The  teeth  of  the  bloodthirsty  white  shark  (Carch;u-ias) 
are  tx'iangular  flattened  plates,  rounded  on  their  posterior 
aspect,  with  trenchant  slightly  serrated  edges ;  it  is  pointed 
out  by  Professor  Owen  that  if  the  relation  between  the  size 
of  the  teeth  and  that  of  the  body  were  the  same  in  extinct 
as  in  recent  sharks,  the  dimensions  of  the  teeth  of  the 
tertiary  Carcharodon  would  indicate  the  existence  of  sharks 
as  large  as  wdiales. 

The  intimate  relationship  between  the  teeth  and  the 
dermal  spines,  which  from  the  standpoint  of  development, 
has  been  illustrated  at  page  2  and  page  119,  is  apparent  also 
in  their  histological  structure.  There  are  many  dermal 
spines  to  be  met  with  in  the  sharks,  which  seen  alone  could 


A    MANUAL    OF  DENTAL    ANATOMY. 


not  possibly  be  distinguished  from  teeth,  the  resemblance 
both  in  outer  form,  in  minute  structure,  and  manner  of 
development  being  most  complete.  The  tooth  figured  on 
page  90  is  a  fair  example  of  a  structure  very  common  among 
the  sharks,  viz.,  a  central  body  of  osteo-dentine,  the  outer 
portion  of  which  has  dentinal  tubes  so  fine,  regular,  and 
closely  packed  as  to  merit  the  name  of  hard  unvascular 
•dentine,  and  over  this  again  a  thin  varnish  of  enamel.  C^) 

And  yet  no  observer  from  its  structure  alone  could  feel 
sure  whether  it  was  a  large  dermal  spine,  or  a  tooth.  Dental 
tissues  occur  in  other  parts  of  the  mouths  of  Selachia  than 
upon  the  jaws,  not  only  in  the  embryonic  stages,  but  in  the 
adult.  Thus  Professor  Turner  lias  described  (Proc.  Koy. 
Society,  Edinburgh,  1880),  very  numerous  comb-like  ap- 
pendages 5  inches  long  upon  the  branchial  arches  of  the 
Basking  Shark  (Selache  maxima),  which  apparently  perform 
the  same  function  as  whalebone  in  straining  the  water. 
These  combs  are  formed  of  a  variety  of  dentine  C?  osteo- 
dentine),  and  closely  resemble  in  structure  the  true  teeth, 
which  are  however  very  small  in  this  shark. 

In  the  seas  of  Australia  there  exists  a  Shark,  the 
Cestracion  Philippi,  with  a  very  aberrant  dentition,  to  which 
great  interest  attaches,  inasmuch  as  it  is  the  sole  surviving 
representative  of  forms  once  spread  all  over  the  world.  In 
the  front  of  the  mouth  the  teeth  are  small  and  very 
numerous ;  they  are  flat  plates  fitted  by  their  edges  to  one 
another,  while  from  their  centres  spring  up  sharp  points, 
soon  worn  oif  when  the  tooth  reaches  such  a  position  upon 
the  jaw  that  it  comes  into  use. 

Proceeding  backwards,  the  teeth  cease  to  be  pointed, 
increase  in  size,  and  become  fewer  in  each  row ;  a  reference 
to  the  figure  will  convey  a  better  idea  of  their  general  form 
than  any  description.  Those  which  have  come  into  use 
are,  towards  the  back  of  the  mouth,  always  much  worn ; 
their  shedding  and  renewal  takes  places,  as  in  other  sharks, 


THE    TEETH    OF  FISHES.  221 


by  a  rotation  of  the  mucous  membrane  over  the  surface 
of  the  jaw,  so  that,  as  might  have  been  expected,  large 
numbers  of  the  isolated  fossil  teeth  of  Cestracionts  are  to  be 
met  with. 

The  teeth  of  the  Cestracion  are  fitted  for  the  trituration 
of  hard  substances,  and  for    such   they  are  used,  its  food 


consisting  of  shell-fish,  etc.  The  teeth  consist  of  vaso-  and 
osteodentine,  protected  by  what  is  appai-ently  a  structure- 
less layer  of  enamel. 

The  extinct  Cestracionts  extended  far  back  in  time,  beinf 
met  with  in  palaeozoic  strata,  and  they  were  equally  widely 
distributed  in  space  ;  the  size  of  many  of  the  teeth  also 
indicates  the  existence  of  forms  much  larger  than  the  recent 
timid  and  inoffensive  Cestracion  Philippi.  Many  of  the 
extinct  forms  are  known  only  by  isolated  teeth ;  of  others 

(')  Lower  jaw  of  Cestracion  Philippi.  a.  Young  teeth  not  yet  in  use. 
b.  Large  grinding  back  teeth,     c.   Small  pointed  front  teeth. 

The  new  teeth  are  developed  at  the  bottom  of  the  series  on  tlie  inner 
side,  and,  just  as  in  other  sharks,  are  covered  in  by  a  flap  of  mucous 
membrane. 


222 


A    MANUAL    OF   DENTAL    ANATOMY. 


ijs.s. 


portions  of  tlie  jaw  with  teeth  in  situ  have  been  discovered  ; 
thus  fragments  of  the  jaw  of  Acrodiis, 
Fig.  95  (').  ^^g  isolated  fossil  teeth  of  which  have 

been  compared  to  fossil  leeches,  with 
seven  teeth  arranged  in  series,  have  been 
met  with. 

The  Pristis,  or  Saw  fish,  so  far  as  the 
mouth  is  concerned,  is  in  no  way  re- 
markable, its  teeth  being  small  and 
blunt,  like  those  of  many  rays.  The 
snout  is,  however,  prolonged  to  an 
enormous  length,  and  is  shaped  like  a 
gigantic  spatula,  its  thin  edges  being 
beset  by  dermal  spines  of  large  size, 
arranged  at  regular  intervals,  and  im- 
planted in  distinct  sockets.  These 
dermal  spines,  or  rostral  teeth,  as  they 
are  sometimes  termed,  are  not  shed 
and  replaced,  but  grow  from  persistent 
pulps ;  in  structure  they  closely  re- 
semble the  teeth  of  Myliobates  (see 
page  82),  being  made  up  of  parallel 
denticles,  in  the  centre  of  each  of 
wiiich  is  a  pulp  cavity  or  medullary 
canal. 

What  xise  the  Saw  fish  makes  of  its 
armed  snout  is  not  very  certainly 
known,  but  its  rostral  teeth  are  of 
interest  to  the  odontologist  for  several 
reasons — the  one  that  they  are  dermal 
spines,  having  a  structure  all  but  iden- 


(')  Rostrum  and  under  side  of  the  head  of  a  small  Pristis.  o.  Mouth. 
i.   Rostrum,      c.   One  of  the  rostral  teeth. 

The  teeth,  with  which  the  margins  of  the  jaws  are  covered,  are  so  small 
that  they  cannot  be  represented  in  this  figure. 


THE    TEETH    OF   FISHES. 


tical  with  that  of  the  actual  teeth  of  another  ray,  the 
Myliobates ;  the  other  that  they  are  socketed,  a  manner  of 
implantation  not  at  all  common  amongst  the  teeth  of  fishes  ; 
and  yet  another,  that  they  grow  from  persistent  pulps,  also 
unusual  in  fishes. 

Broadly  speaking,  the  teeth  of  the  Kays  (skates)  differ 
from  those  of  typical  sharks  by  being  individually  blunter, 
and  being  more  closely  set  so  that  they  foiTu  something  ap- 
proaching to  a  contniuous  pavement  over  the  jaws,  with  but 
little  interspace  left  between  the  teeth. 


Fig.  96  ('). 


:N.S 


The  dentigerous  surface  of  the  jaw  is  very  much  rounded, 
and  in  some  is  completely  encased  under  a  pavement  of  teeth. 


(^)  Upper  and  lower  jaw  of  Myliobates.  At  a,  the  mosaic  pavement 
fonned  by  the  broad  flattened  plates  whioli  constitute  its  teeth  is  seen, 
these  being  the  oldest  teeth  which  are  about  to  be  shed  off  in  consequence 
of  the  rotation  of  the  whole  sheet  of  mucous  membrane  over  the  surface 
of  the  jaws.  The  letter  6  indicates  the  under  surface  of  one  of  the  plates, 
which  is  seen  to  be  finely  fluted  on  its  edge. 


224  A    MANUAL    OF  DENTAL    ANATOMY. 


Thus,  in  Myliobates,  the  powerful  jaws  are  straight  from  side 
to  side,  while  their  working  surfaces  from  back  to  front  are 
segments  of  a  circle.  The  teeth  form  a  thick  and  strong 
pavement  over  the  jaws,  in  the  manner  of  their  formation 
and  renewal  conforming  with  the  teeth  of  other  Plagiostomi  ; 
the  severe  use  to  which  they  are  put  being  indicated  by  the 
extent  to  which  the  gi-inding  surfaces  of  those  teeth  which 
have  come  into  use  are  worn  down. 

Several  genera  have  the  jaws  thus  covered,  the  number 
of  the  teeth  differing  ;  thus  Myliobates  has  a  central  series 
of  very  broad,  oblong  teeth,  to  the  outer  sides  of  which  are 
three  rows  of  small  hexagonal  teeth  ;  in  (Etobatis  the  large 
oblong  central  plates  constitute  the  whole  armatiu-e  of  the 
jaw. 

The  structure  of  the  teeth  of  Myliobates  has  already  been 
described  and  figured  (see  page  82). 

The  Teleostei,  or  osseous  fish,  form  the  group  which  Com- 
prises all  the  fish  most  familiarly  known  to  us,  and  within 
its  limits  the  variation  in  dentitions  is  so  gi-eat  that  few,  if 
any,  general  statements  can  be  made  about  them.  It  is  not 
uncommon  to  find  teeth  crowded  upon  every  one  of  the 
bones  which  form  a  part  of  the  bony  framework  of  the 
mouth  and  pharynx,  and  the  teeth  are  sometimes  in  count 
less  numbers.  And  so  great  is  the  variability  that  even 
within  the  limits  of  single  families  differences  in  the  teeth 
are  to  be  found. 

In  the  common  pike  the  mouth  is  crowded  with  sharply- 
pointed  teeth,  having  a  general  inclination  backwards,  and 
being  in  some  parts  of  the  mouth  of  larger  size  than  in 
others.  The  margin  of  the  lower  jaw  is  armed  with  teeth  of 
formidable  size  and  sharpness,  the  smallest  teeth  being  at 
the  front,  where  they  are  arranged  in  several  rows,  and  the 
largest  being  about  the  middle  of  the  side  of  the  jaw,  A 
pike,  as  is  well  known  to  anglers,  when  it  has  seized  a  fish 
often  holds  it  across  its  mouth,  piercing  and  retaining  it  by 


THE    TEETH    OF   FISHES. 


means  of  these  largest  teeth  ;  theu,  after  holding  it  thus  for 
a  time,  and  so  maimed  it  and  lessened  its  power  of  escape,  it 
swallows  it,  generally  head  foremost.  The  tenacity  of  the 
pike's  hold  is  often  illustrated  when  it  takes  a  bait,  and 
retains  it  so  firml}-  that  when  the  angler  '•'  strikes "  the 
hooks  do  not  get  driven  into  the  fish's  mouth ;  but  aftei- 
tugging  at  the  bait  for  a  time  the  pike  releases  it,  and  the 
angler  finds  that  it  has  never  been  hooked  at  all. 

The  margin  of  the  upper  jaw  is  not  bordered  by  tectli, 
save  at  the  front,  where  the  intermaxillary  bones  carry  a 
few  teeth  of  insignificant  dimensions  ;  indeed,  it  is  rather 
exceptional  for  the  true  maxillary  bones  to  carry  teeth  in 
osseous  fish.  The  roof  of  the  mouth  presents  three  Avide 
parallel  bands  of  teeth,  those  in  the  median  band  (on  the 
vomer)  being  directed  backwards,  those  upon  the  lateral 
bands  (on  the  palatine  bones)  backwards  and  inwards. 
Some  of  the  latter  teeth  are  very  large,  but  not  quite  so 
large  as  those  at  the  sides  of  the  lower  jaw. 

The  marginal  teeth  are  firmly  anchylosed,  but  the  teeth 
upon  the  palate  are  all  hinged,  and  in  such  a  manner  that 
they  can  only  bend  exactly  in  one  direction.  Those  of  the 
vomerme  band  which  lie  in  the  middle  line,  will  bend  back- 
wards only  ;  those  upon  the  outer  margins  of  this  band 
backwards,  with  an  inclination  outwards.  Those  of  the 
lateral  or  palatine  bands  bend  obliquely  backwards  and 
inwards,  about  at  an  angle  of  45  with  the  median  line  of 
the  mouth,  or  somewhat  more  directly  backwards.  To  a 
body  sliding  over  them  in  -  one  direction  they  ofter  no 
resistance,  bending  down  as  it  passes,  and  sjjringiug  up  as 
the  pressm-e  is  removed  from  them,  but  to  anything  moving 
in  any  other  direction  they  are  rigidly  fixed  sharp  curved 
stakes  impeding  its  further  progress. 

An  elongated  body  of  some  size,  such  as  a  living  fish, 
can  only  be  swallowed  by  the  pike  when  it  is  arranged 
lengthwise  in  the  mouth ;  crosswise  it  cannot  possibly  enter 

Q 


2-20 


A    MANUAL    OF   DENTAL    ANATOMY. 


the    throat.     The   hinged   teeth    on   the    paLate    seem  ad- 
mirably arranged  for  getting  the  fish   into  a  longitudinal 


position  and  keeping  it  there  ;  for,  if  we  imagine  the  fish's 

(^)  Jaws  of  a  Pike,  viewed  from  the  front,  with  the  mouth  opened  more 
widely  than  is  natural,  so  as  to  bring  the  teeth  into  view.  a.  Group  of 
teeth  situated  on  the  pahitine  bone.  h.  Group  of  teeth  situated  on  the 
vomer,  c.  Group  of  teeth  situated  on  the  lingual  bone.  d.  Specially 
large  teeth,  placed  at  intervals  round  the  margin  of  the  lower  jaw. 
e.   Group  of  teeth  on  the  intermaxillary  l)ones. 

The  diagram  beneath  represents  the  direction  in  which  the  hinged  teeth 
of  the  vcmerinc  and  palatine  bands  can  bci;d. 


THE    TEETH    OF   FISHES.  221 

body  held  up  against  these  teeth,  and  consider  the  direction 
in  which  the  hinging  of  the  teeth  allows  them  to  yield,  it 
will  be  seen  that  every  motion  tending  to  arrange  the  body 
lengthwise,  either  in  the  median  line  of  the  mouth  or  in 
either  of  the  interspaces  between  the  vomerine  and  palatine 
bands  of  teeth,  will  meet  with  no  obstruction,  but  in  eveiy 
deviation  from  this  position  it  will  be  caught  on  the  points  of 
the  teeth  and  resisted.  Thus  with  the  pike's  mouth  shut, 
and  the  fish  kept  up  against  the  palatine  teeth,  even  its  own 
struggles  will  be  utilised  by  every  movement  tending  to 
place  it  aright  being  allowed,  and  every  other  stopped  by  the 
bands  of  hinged  teeth  entangling  it. 

The  structure  of  these  teeth,  and  the  mechanism  by  which 
they  are  rendered  elastic,  have  been  already  described 
(page  206). 

The  lingual  bone,  and  the  three  median  bones  behind  it, 
carry  small  teeth  arranged  in  oblong  patches  ;  the  internal 
surfaces  of  the  branchial  bones  (which  support  the  gills) 
are  armed  with  similar  small  teeth ;  while  the  last  or  fifth 
branchial  arch  (which  carries  no  gills,  the  bones  forming 
it  being  called  inferior  pharyngeal  bones,)  carry  larger 
teeth.  The  superior  pharyngeal  bones  (which  are  median 
portions  of  the  four  anterior  branchial  arches)  also  carry 
recurved  teeth  larger  than  those  which  line  the  rest  of  the 
internal  surfaces  of  each  of  the  branchial  arches. 

The  pike's  mouth  and  pharynx  thus  fairly  bristle  with 
teeth,  all  directed  somewhat  backwards ;  and  any  one  who 
has  been  unfortunate  enough  to  have  allowed  his  fingers 
to  get  entangled  in  the  mouth  of  a  living  pike  will  realise 
how  small  a  chance  its  living  prey  has  of  escape,  when  once 
it  has  been  seized. 

The  teeth  of  the  pike  are  composed  of  a  central  body  of 
osteo-dentine,  on  the  outside  of  which  is  a  layer  in  which 
the  dentinal  tubes  are  directed  towards  the  surface,  as  in 
hard  or  unvascular  dentine  ;  while  the  outermost  portion  of 


228  A    MANUAL    OF   DENTAL    ANATOMY. 

all  is  a  very  dense  and  hard,  and  apparently  structureless, 
enamel  film.  The  teetli  are  anchylosed  to  the  bone,  and  are 
very  frequently  renewed,  their  successors  being  developed  at 
one  side  of  their  bases. 

Though  the  pike  has  rather  more  teeth  than  many  other 
fish,  it  may  be  taken  as  a  fair  example  of  most  osseous  fishes 
in  this  respect.  Space  will  only  allow  of  a  few  of  the  more 
exceptional  forms  being  here  described. 

The  angler  (Lophius  piscatorius),  another  predatory  fish, 
with  an  enormous  mouth  and  disproportionately  small  body 
and  tail,  lies  hidden  in  the  mud,  or  crouched  upon  the 
bottom,  and  makes  a  rush  upon  smaller  fishes  which  ap- 
j)roach  sufficiently  near  to  it ;  it  is  remarkable  for  the 
manner  of  attachment  of  the  teeth,  some  of  the  largest  of 
which  upon  the  edges  of  its  jaws  do  not  become  anchylosed, 
but  are  so  attached,  as  has  been  described  at  p,  20-3,  as  to 
iillow  of  their  bending  in  and  towards  the  mouth,  but  not  in. 
the  opposite  or  any  other  direction.  The  teeth  of  the  outer 
row  are  firmly  anchylosed  to  the  margins  of  the  jaw,  and  the 
far  larger  hinged  teeth  form  a  sort  of  irregular  second  row. 

The  benefit  of  such  an  arrangement  to  a  fish  of  its  habit 
is  sufliciently  shown ;  its  teeth  allow  the  utmost  freedom  of 
entry,  but  offer  obstacles  to  anything  getting  out  again. 

This  arrangement  of  teeth,  long  supposed  to  be  unique,  is 
closely  paralleled  in  a  very  difterent  fish,  the  Hake  (Mer- 
lucius,  one  of  the  Gadicke).  This  fish,  the  most  active  and 
l^redatory  of  the  Cod  family,  follows  shoals  of  pilchards  and 
of  herrings,  themselves  active  fish,  and  feeds  upon  them.  The 
margins  of  the  jaws  carry  two  distinct  and  regularly  ar- 
ranged rows  of  teeth,  an  outer  smaller  row  which  are 
anchylosed,  and  an  inner  longer  row  which  are  hinged. 
They  are  very  sharp,  being  tipped  with  spear  points  of 
enamel,  and  are  recurved.  In  the  fresh  state  they  look 
quite  red,  being  composed  of  a  riclily  vascular  vaso- 
deutine. 


THE    TEETH    OF   FISHES. 


229 


Another  curious  dentition  is  possessed  by  the  Wolf-fish 
(Anarrhicas  lupus),  also  an  inhabitant  of  British  waters, 
and  sometimes  to  be  seen  in  London  fishmongers'  shops 


under  the  name  of  the  sea  cat.  The  intermaxillary  teeth 
are  conical,  bluntly  pointed,  and  set  forwards  and  outwards  ; 
these  are  antagonised  by  somewhat  similar  teeth  in  the 
front  of  the  lower  jaw.  The  palatine  bones  carry  short, 
bluntly  conical,  or  round  tupped  crushing  teeth  in  a  double 

(')  Bones  of  the  mouth  of  the  Wolf-fish  (Anarrhicas  lupus).  The  letter 
a.  indicates  the  divergent  pointed  teeth  which  occupy  the  intermaxillaiT 
bone  ;  the  letter  d.  indicates  the  similar  teeth  which  are  attached  to  the 
front  of  the  mandible,  on  the  middle  and  back  parts  of  which  are  round- 
topped  crushing  teeth  (c).  Strong  crashing  teeth  are  found  also  upon  tlie 
I>alatine  bones  (i),  and  upon  the  vomer  (c). 


230  A    MANUAL    OF   DENTAL    ANATOMY. 


row ;  the  vomer  is  also  armed  with  a  double  row  of  very 
much  larger  and  shorter  teeth;  the  lower  jaw,  with  the 
exception  of  its  anterior  part,  is  occupied  by  teeth  of  similar 
character. 

All  tlic  teeth  of  the  Wolf-fish  are  anchylosed  slightly  to 
the  bone,  a  definite  process  from  which  forms  a  sort  of  short 
pedestal  for  each  tooth.  The  jaws  are  worked  by  muscles 
of  gi'eat  power,  and  it  seldom  happens  that  a  specimen  is 
examined  in  which  some  of  the  teeth  are  not  broken.  It 
feeds  upon  shell  fish,  the  hard  coverings  of  which  are 
crushed  by  the  blunter  teeth,  while  the  pointed  front  teeth 
apparently  serve  to  tear  the  shell  fish  from  the  rocks  to 
which  they  are  commonly  attached. 

In  the  group  of  fish  known  as  "  Gj'mnodonts "  {naked 
toothed),  the  teeth  and  the  margins  of  the  dentigerous  bones 
form  a  sort  of  beak,  which  is  not  covered  by  the  lips.  The 
example  here  figured  consists  of  the  upper  and  lower  jaws  of 
the  Diodon,  so  called  because  it  appears  to  casual  observators 
to  have  but  two  teeth.  A  kindred  fish  in  which  the  division 
of  each  jaw  in  the  middle  line  is  conspicuous,  is  similarly 
called  Tetrodon.  The  jaw  consists  of  teeth  and  bone  very 
intimately  fused  together;  the  broad  rounded  mass  (c.  in 
the  figure),  which  lies  just  inside  the  margin  of  the  jaws,  is 
made  up  of  a  number  of  horizontal  plates  of  dentine,  the 
edges  of  which  crop  out  upon  its  posterior  surface ;  and 
these  are  united  to  one  another  by  the  calcification  of  the 
last  remains  of  the  pulp  of  each  plate  into  a  sort  of  osteo- 
dentine,  the  different  hardness  of  the  two  tissues  keeping 
the  surface  constantly  rough,  as  the  plates  become  worn  away. 
The  whole  margin  of  the  jaw  is  similarly  built  up  of  smaller 
horizontally  disposed  denticles,  or  jDlates  of  dentine,  which 
are,  as  they  wear  down,  replaced  by  the  development  of 
fresh  plates,  which  are  added  from  beneath,  where  they  are 
developed  in  cavities  situated  low  down  in  the  substance  of 
the  bone. 


TEE    TEETH    OF  FISHES. 


231 


The  new  teeth  or  plates  of  dentine  thus  formed  at  the 
base  of  the  hemispherical  masses  -within  the  jaws  (at  the 
point  a),  or  low  down  in  the   sulistance  of  the  jaw,  do  not 


Fig.  99  ('). 


3\  S 


come  into  use  by  the  ordinary  process  of  displacing  their 
predecessors,  and  being  in  turn  themselves  replaced,  but 
fresh  plates  only  come  into  use  by  the  actual  wearing  a-way 
of  all  that  is  above  them,  both  dentine  and  bone,  so  that 
they  come  to  be  the  topmost  portion  of  the  jaw.  The 
margins  of  the  jaw  are,  however,  mainly  built  up  of  dental 
tissues,  there  being  but  little  bone  in  their  interspaces. 

Tetrodou  has  not  the  roxmded  triturating  disk  of  the 
Diodon,  or  has  it  but  feebly  rcpi-eseutcd ;  and  the  margins 
of  the  jaws  are  sharpen 

In  the  Parrot-fishes  (Scai'us),  which  are  not  very  nearly 
allied  to  the  Gymnodonts,  somewhat  similar  beaks  are  found, 


(')  Jaws  of  the  Diodon.  a.  Base  of  tlie  dental  plates,  where  new  lamellm 
of  dentine  are  being  developed,  h.  Margin  of  jaw,  formed  mainly  by  the 
sides  of  the  denticles,  c.  Compound  tooth,  made  up  of  the  superimposed 
lamelloe  of  dentine  anchylosed  together. 


232  A    MANUAL    OF   DENTAL    ANATOMY. 

the  individual  teeth  being  more  conspicuous.  The  whole 
outer  surface  of  the  jaw  near  to  its  working  edge  is  covered 
by  a  sort  of  tesselated  pavement,  formed  by  the  several 
teeth  which  are  pressed  together  into  a  mass,  but  they  form 
only  the  outer  surface  and  the  immediate  edge,  so  that  the 
soft  bone  forms  a  part  of  the  working  surface,  or  would  do 
so  but  that,  by  its  more  speedy  wear,  it  leaves  the  edge, 
formed  by  dentine  and  enamel,  always  i)romincnt  and  more 
or  less  sharp. 

The  structure  and  succession  of  these  teeth  have  been  care- 
fully described  by  J.  von  Boas  (Zeits.  f.  Wissen.  Zool.  xxxii.), 
and  the  differences  between  the  several  genera  pointed  out. 
He  describes  cementum  as  binding  the  denticles  together 
and  forming  a  part  of  the  working  edge,  but  that  which  he 
describes  as  cementum  appears  to  me  to  be  that  tissue 
which  I  have  termed  "  bone  of  attachment."  See  page  208. 
In  a  section  of  a  jaw  in  my  possession,  which  I  believed 
to  have  belonged  to  a  Gymnodont  fish  but  which  bears  a 
remarkably  close  resemblance  to  that  figured  by  von  Boas 
as  being  a  jaw  of  Pseudoscarus,  a  very  beautiful  arrange- 
ment serves  to  preserve  the  sharpness  of  the  edge  of  the  jaw. 
The  denticles  are  conical,  and  form  a  series  of  hollow 
superimposed  cones  with  the  points  upwards ;  they  consist 
of  dentine  and  enamel,  and  the  jDoint  of  the  subjacent  cone 
fits  closely  up  into  the  hollow  of  that  above  it,  so  closely 
that  in  von  Boas'  specimen  the  dentine  of  the  older  tooth  is 
in  great  part  absorbed  to  make  way  for  the  point  of  its 
successor,  so  that  the  working  denticle  comes  to  be  little 
more  than  a  hollow  cone  of  enamel.  This  is  not  the  case  in 
my  specimen  in  which  there  is  a  quantity  of  dentine  left 
in  each  denticle.  This  vertical  series  of  superimposed 
sharp  cones  lie  in  the  midst  of  the  somewhat  thin  jaw  bone, 
fused  together  by  cementum  (J  bone  of  attachment),  and 
enclosed  between  the  inner  and  outer  plates  of  the  jaw. 
The  bone   being  much   softer    than    the    denticle,  wears 


THE    TEETH    OF    FISHES.  233 

down  much  faster,  so  that  the  edge  is  always  formed  by  a 
prominent  sharp  tooth,  which,  as  the  wearing  down  of  the 
bone  progresses,  falls  off,  and  the  next  one  beneath  it  comes 
into  play.  The  arrangement  recalls  the  way  in  which  a 
scythe  or  a  chisel  is  assisted  in  keeping  its  edge  by  being 
made  of  a  plate  of  steel  welded  between  two  plates  of 
softer  iron. 

The  pharyngeal  bones  are  also  remarkable;  the  two  lower 
are  united  into  one,  and  the  stout  bone  so  formed  is  armed 
with  teeth ;  it  is  antagonised  by  two  upper  pharyngeal  bones 
similarly  armed.  It  carries  teeth  which  are  anchylosed  to 
it,  and  which  are  so  disposed  as  to  keep  the  surface  con- 
stantly rough.     When   they   are  freshly  formed   the  teeth 

Fig.  100  (i). 


,r 


c- 


have  flattened  thin  edges,  something  like  human  incisors. 
The  teeth  are  coated  with  enamel,  and  thus,  Avhen  calcifi- 
cation has  proceeded  so  far  as  to  obliterate  their  central 
pulp  cavities,  after  the  tooth  is  worn  to  a  certain  point 
(c  in  Fig.  99)  it  presents  a  ring  of  enamel,  inside  which 
comes  a  ring  of  dentine,  and  inside  this  a  core  of  secondary 
dentine,  as  seen  in  the  figure.     Owing  to  the  different  hard- 

(•)  Lower  pharjTigeal  bone  of  Pseudoscarus.  a.  Posterior  Ijorder,  at 
Avhich  the  teeth  are  umvorn.  c.  Oval  areas  formed  by  teeth,  the  points 
of  which  are  worn  off.  b.  Anterior  edge  of  bone,  at  which  the  teeth  ai'c 
iilmost  completely  worn  away. 


234  A    MANUAL    OF  DENTAL    ANATOMY. 


ucss  of  the  three  tissues  a  constant  roughness  of  surface  is 
maintained.  The  upper  pharyngeals  are  similarly  armed  ; 
and  as  the  teeth  and  the  supporting  bone  wear  away,  fresh 
teeth  are  developed  at  the  front,  so  that  the  whole  bone 
undergoes  a  sort  of  gliding  motion  backwards,  the  armature 
of  the  lower  pharyngeal  being  renewed  in  a  similar  manner, 
save  that  new  teeth  and  bone  are  developed  at  its  j)osterior 
instead  of  its  anterior  extremity. 

The  teeth  are  developed  in  bony  crypts,beyond  the  youngest 
functional  teeth,  and  perforations  in  the  roofs  of  the  crypts 
give  passage  to  the  connecting  band  l^etween  the  tooth  sac 
and  the  mucous  membrane. 

No  more  fitting  place  will  occur  for  noticing  the  stout 
pharyngeal  teeth  which  are  met  with  in  so  many  fish. 
Some  fish,  which  are  edentulous  so  far  as  the  mouth  is 
concerned,  have  the  j)haryngeal  bones  armed  ■s\'itli  teeth  ; 
in  the  carp  and  its  allies,  edentulous  so  far  as  the  mouth 
proper  is  concenied,  the  two  lower  pharyngeal  bones  carry 
long  pointed  teeth,  which  partly  oppose  one  another,  and 
partly  oppose  a  sort  of  horny  tubercle,  which  is  supported 
on  a  process  of  the  base  of  the  occipital  bone. 

A  few  fish  are  quite  without  teeth ;  the  sturgeon,  whose 
mouth  forms  a  protrusible  sucker,  is  edentulous,  as  are  also 
the  pipe  fish,  and  the  little  sea  horse  (Hippocampus),  now  so 
common  in  aquaria. 

But  as  a  rule  fish  are  remarkable  for  the  great  number  of 
their  teeth,  which  are  being  constantly  shed  off  and  replaced 
by  successors  an  indefinite  number  of  times. 

In  all  the  fish  hitherto  mentioned  in  these  pages,  it 
happens  that  the  teeth  in  different  parts  of  the  mouth  difi"er 
in  size  and  in  the  function  which  they  have  to  perform ; 
but  this  is  only  so  because  a  few  striking  forms  have  been 
naturally  selected  fur  description.  It  is  far  commoner  for 
all  the  teeth  of  fish,  particularly  of  those  fish  which  have 
countless  numbers  of  teeth,  to  be  very  nearly  alike  in  form 


THE    TEETH    OF  FISHES.  235 


and  size  iu  all  parts  of  the  mouth.  As  a  general  rule,  fish 
do  not  comminute  their  food  very  fully,  but  make  use  of 
their  teeth  simply  for  the  apprehension  of  prey,  not  sub- 
mitting the  food  to  any  mastication  whatever ;  their  teeth 
are  hence  often  mere  sharp  cones,  slightly  recurved,  or  set 
looking  backwards.  Thus,  though  the  mouth  of  the  common 
pike  is  beset  with  an  immense  number  of  sharp  teeth,  its 
food  is  swallowed  whole,  and  ver}'  often  is  alive  w^hen  it 
reaches  the  stomach,  the  sole  purpose  served  by  the  teeth 
being  the  prevention  of  its  escape  when  once  it  has  been 
seized. 

Implantation  of  the  teeth  in  sockets  is  not  usual  in  the 
class  of  fish,  but  it  does  occur  :  for  example  the  Barracuda 
pike  (Sphymena)  has  its  lancet-shaped  teeth  implanted  in 
distinct  sockets,  to  the  walls  of  which  they  are  said  to 
become  slightly  anchylosed;  the  file-fish  and  others  might 
also  be  cited.  And  although  the  succession  of  teeth  is 
usually  from  the  side,  in  some  cases  the  successional  teeth 
are  developed  in  alveolar  cavities  within  the  substance  of 
the  bone,  and  displace  their  predecessors  in  a  vertical  direc- 
tion, as  happens  in  the  pharyngeal  teeth  of  the  Wrasses,  or 
the  curiously  human-looking  incisors  of  the  Sheep's-head  fish 
(Sargus) ;  the  Lepidosteus  also  has  its  teeth  aflixed  in  incom- 
plete sockets,  to  the  walls  of  which  they  are  anchylosed  ; 
this  is  not  a  very  uncommon  airaugement  with  the  teeth  of 
fish  when  they  are  socketed  at  all. 

The  teeth  of  fish  are  of  all  degrees  of  size  and  of  fineness  ; 
in  some  (Chtetodonts).  the  teeth  are  as  fine  as  hairs,  and  arc 
so  soft  as  to  be  flexible. 

Teeth  which  are  very  fine  and  very  closely  set  are  termed 
"dents  en  velom-s;"  when  they  are  a  little  stouter,  "dents 
en  brosse,"  and  when  still  stronger  and  sharper,  "dents  en 
cardes."  Teeth  that  are  conical,  wedge-shaped,  spheroidal, 
and  lamelliform,  are  all  to  be  met  with;  in  fact  there  is 
infinite  diversity  in  the  form  of  fishes'  teeth. 


A    MANUAL    OF  DENTAL    ANATOMY. 


And  there  are  some  fish,  e.g.,  some  of  the  large  Siluroicl 
fishes,  which  have  very  strong,  large  teeth,  an  inch  and  a 
half  or  more  long,  and  very  firmly  anchylosed  to  the  bone. 

It  is  not  common  for  sexual  differences  to  be  met  with 
between  the  teeth  of  the  male  and  female,  though  a  slight 
difference  exists  between  the  sexes  in  some  species  of  Skate. 
And  although  not  strictly  speaking  a  dental  character,  it 
may  not  be  out  of  place  to  mention  here  the  peculiar 
armature  of  the  jaw  of  the  male  Salmon  at  the  breeding 
season. 

The  end  of  the  lower  jaw  becomes  produced,  and  turned 
upwards  at  its  point;  the  stout  cartilaginous  hook  thus 
formed  is  of  such  dimensions  that  it  has  to  be  accommo- 
dated in  closure  of  the  mouth  in  a  deep  cavity  formed 
for  it  l)etwcen  the  intermaxillary  bones.  In  some  Canadian 
salmon  this  process  is  supposed  to  be  constant  in  the  older 
males,  but  in  the  British  fish  it  disappears,  and  only  exists 
at  the  breeding  season,  A  fish  in  which  it  is  strongly 
developed  is  a  foul  fish,  and  is  called  a  Kelt.  It  is  used 
apparently  as  a  battering  ram,  and  such  salmon  are  con- 
stantly found  killed,  with  their  sides  deeply  gashed  by  the 
charges  of  their  opponents. 

jS'ot  much  can  be  said  in  general  terms  of  the  structure 
of  the  teeth  of  fish.  The  bulk  of  the  teeth  of  most  fishes 
is  made  up  of  one  or  other  modification  of  vasodentine  or 
osteodentine ;  this  is  often  glazed  over  upon  its  exterior  by 
a  thin  film  of  enamel,  so  thin  as  often  to  appear  structure- 
less. 

Unvascular  dentine  also  forms  the  teeth  of  many  fish, 
and  in  some  is  remarkable  for  the  fineness  of  its  tubes ;  in 
fact,  every  form  of  dentine,  from  fine-tubed  hard  dentine 
to  tissue  indistinguishable  from  coarse  bone  is  to  be  found 
in  this  class. 

Dentine  of  very  complex  structure  (labyrintho-dentine) 
is  met  with  in  some  fish ;  and  an  example  from  the  Lepi- 


TKE    TEETH    OF  FISHES.  237 

dosteus  (Americau  garpike,  a  ganoid  fish)  has  been  figured  at 
page  78. 

Enamel  is  often  present  in  a  very  thin  layer,  glazing  the 
exterior  of  the  dentine  (see  Fig.  48)  ;  sometimes  it  forms  a 
mere  tip,  a  sort  of  spear-point  to  the  tooth  as  in  the  Eel  and 
the  Hake  (see  Figs,  87  and  90),  and  sometimes  it  is  very 
thick,  and  itself  permeated  by  systems  of  tubes  (see  Fig.  24). 

Cementum  is  of  comparatively  rare  occurrence  in  fish. 

Professor  Kolliker  has  shown  that  in  a  very  large  number 
of  fishes  the  skeleton  more  nearly  resembles  dentine  than 
true  bone  in  its  structure  ;  whilst  the  dermal  scales  and 
protective  spines  of  fish  are  often  made  up  of  a  tissue  much 
resembling  dentine  (cf  Professor  "Williamson,  Philos.  Trans. 
1849).  AVe  may  say,  then,  that  just  as  in  the  external  skin, 
bony  or  dentinal  plates  are  developed  for  the  purpose  of 
protecting  it  from  destruction  by  attrition,  so  for  a  similar 
purpose  teeth  are  developed  in  that  portion  of  the  mucous 
membrane  which  covers  the  jaws. 

Near  the  borderland  between  fish  and  amphibia  is  the 
Lepidosiren,  or  ^Mud-fisli,  which  is  a  fish  rather  than  an 
amphibian.  The  armature  of  its  mouth  is  peculiar,  the 
margins  of  the  lower  jaws  being  formed  by  dental  plates 
anchylosed  to  the  bone.  These  plates  have  upon  their 
edges  five  deep  angular  notches,  the  prominence  of  the 
upper  plate  con-esponding  to  the  notches  of  the  lower  ; 
and  the  edge  is  kept  somewhat  sharp  by  the  front  surface 
being  formed  of  very  dense  hard  dentine,  while  the  bulk 
of  the  tooth  is  permeated  by  large  medullary  canals,  which 
render  it  softer.  The  cutting  plates  of  the  upper  jaw  are 
developed  in  tlie  median  line  of  the  palate,  and  there  arc 
in  front  of  them  conical  piercing  teeth  upon  that  forward 
prolongation  of  the  cartilage  which  takes  the  place  of  a 
distinct  vomer ;  these  have  sometimes  been  described  as 
being  upon  the  nasal  bone. 

It  would  seem  that  the  two  conical  piercing  teeth  serve 


A    MANUAL    OF   DENTAL    ANATOMY. 


as  holdfosts,  while  the  cutting  edges  of  the  deeply-notched 
plates  are  brought  into  play  to  slice  np  the  food. 

Both  in  structure  and  general  disposition  the  dental  plates 
in  Lepidosiren  arc  paralleled  by  the  teeth  of  Ceratodus,  for 
some  time  known  only  as  a  fossil,  but  of  which  recent 
examples  have  been  captured  near  Queensland ;  this  re- 
semblance was  suspected  some  years  ago  by  my  friend, 
i\Ir.  Moseley,  of  the  Challenger,  and  has  been  since  worked 
out  by  other  observers. 


CHAPTER  VII. 

THI:;    TEETH    OF    BATRACHIA    AXD    RErTILES. 

In  these  classes  the  teeth  are  never  so  numerous  nor  so 
widely  distributed  upon  the  bones  of  the  mouth  as  in  iish ; 
a  double  row  of  teeth  arranged  in  concentric  lines  in  the 
upper  jaw,  between  which  a  single  row  of  teeth  upon  the 
lower  jaw  passes  when  the  mouth  is  closed,  is  an  arrange- 
ment rather  common  amongst  Batrachia.  Almost  all 
Batrachians  and  Reptiles  have  an  endless  succession  of 
teeth ;  but  there  are  a  few  lizards  {e.g.,  Hatteria),  in  which 
the  manner  of  succession,  if  there  be  any,  has  not  l)een 
definitely  ascertained.  The  outer  of  the  two  rows  of  teeth 
in  the  upper  jaw  is  situated  upon  the  premaxillary  and 
maiillary  bones,  and  usually  extends  further  back  than 
the  vomerine  or  inner  row. 

From  this  type  of  dentition  there  are  many  deviations  ; 
thus  the  toads  are  edentulous,  and  the  frog  has  no  teeth  in 
the  lower  jaw. 

The  teeth  of  the  frog  form  a  single  row  upon  the  margin 
of  the  upper  jaw,  their  jioints  projecting  but  little  above 
the  surface  of  the  mucous  membrane,  and  the  vomerine 
teeth  are  few  in  number  and  cover  only  a  small  space. 

The  edentulous  lower  jaw  passes  altogether  inside  the 
row  of  upper  teeth,  and,  itself  having  rounded  surfaces 
and  no  lip,  fits  very  closely  against  the  inner  sides  of  the 
teeth.  Thus  it  leaves  very  little  room  for  the  young 
developing  tooth  sacs,  which   are    accommodated  with  the 


A    MANUAL    OF  DENTAL   ANATOMY. 


space  required  for  the  attainment  of  their  full  size,  by  the 
absorption  of  the  older  solid  bone  and  the  tooth  which 
has  preceded  them,  in  the  following  manner.  The  teeth 
are  attached  to  the  Iwne  by  anchylosis,  each  tooth  being 
perched  upon  a  little  pedestal  of  bone  which  is  specially 
formed  for  it ;  and  the  successional  teeth,  the  germs  of 
which  originally  lay  at  the  inner  sides  of  the  old  teeth, 
commonly  undermine  the  side  of  the  pedestals  and  the 
bases  of  the  latter,  and  move  bodily  beneath  them,  so  that 
the  new  tooth  completes  its  development  in  what  was  once 
the  pulp  cavity  of  its  predecessor. 

The  teeth  of  the  frog  consist  of  a  body  of  hard  dentine, 
coated  with  an  exceedingly  thin  layer  of  enamel,  the  exist- 
ence of  which  has  been  doubted  by  some  writers ;  but  a 
study  of  the  tooth  sac  of  the  animal  renders  it  probable  that 
the  transparent  layer  which  is  xnidoubtedly  there  is  really 
enamel. 

The  teeth  of  the  newt  and  its  ally  the  salamander  are 
remarkable  for  having  tips  of  enamel,  somewhat  like  those 
of  the  eel  (see  Fig.  90),  save  that  they  are  bifurcated,  the 
one  point  being  larger  and  longer  than  the  other. 

The  tadpole  has  its  jaws  armed  with  tough  horny  plates 
something  like  a  turtle's  bill,  which  are  shed  off,  prior  to 
the  development  of  any  true  teeth;  at  all  events  I  have 
myself  been  xmsiiccessful  in  discovering  any  tooth  germs  at 
the  ^Deriod  Avhen  its  horny  bills  are  still  in  use. 

Some  extinct  batrachia  were  of  large  size  ;  the  Laby- 
rinthodon,  the  structin-e  of  whose  teeth  has  already  been 
described  (page  79),  was  furnished  with  a  marginal  row  of 
teeth  in  the  upper  jaw,  of  which  some  few  were  of  larger 
size  xmd  greater  length  than  the  others.  In  the  lower  jaw, 
the  teeth,  which  are  similar  to  those  of  the  uptper,  are 
disposed  in  some  sense  in  an  incomplete  double  row,  the 
series  of  smaller  teeth  not  being  interrupted  by  the  occur- 
rence of  the  larger  tusks,  but  passing  in  unbroken  series 


THE    TEETH    OF    REPTILES. 


outside  them.  The  Labyrinthodoii  was  possessed  also  of 
palatine  teeth. 

The  teeth  were  auchylosed  to  slight  depressions  or  sockets, 
and  the  successioual  teeth  were  probably  developed,  as  in 
the  frog,  at  the  inner  side  of  the  bases  of  the  teeth  already  in 
position,  as  there  are  no  indications  of  crypts  within  the  bone. 

In  many  reptiles  teeth  are  developed  for  the  merely  tem- 
porary end  of  effecting  an  exit  from  the  egg-shell.  This 
purpose  is  sufficiently  answered  by  the  hard  snout  of  the 
crocodiles,  and  by  a  sort  of  snout  developed  in  Chelonia,  but 
snakes  and  lizards  have  sharp  teeth,  which  afterwards  are 
lost,  developed  on  the  premaxillary  bones  (Owen), 

The  Chelo:^ia,  comprising  the  Tortoises  and  Turtles,  have 
no  teeth,  but  the  margins  of  the  jaws  are  sheathed  in  horny 
cases,  which  are  variously  shaped  in  accordance  with  the 
habit  of  the  animal,  being  sharp  and  thin  edged  in  carnivor- 
ous, and  blunt  and  rugged  in  herbivorous  species. 

Saurian  reptiles  (lizards,  &c.),  have,  as  a  rule,  rather 
simple  teeth,  which  are  confined  to  the  margin  of  the  jaws, 
the  occun-ence  of  palatal  teeth  being  less  usual.  The 
teeth  are  of  various  forms,  being  blunt  and  rounded  in  many 
genera,  whilst  in  others  they  are  long  and  pointed.  They 
are  generally  made  up  of  a  central  body  of  hard  dentine, 
more  or  less  completely  invested  by  a  cap  of  enamel ;  and 
they  are  attached  to  the  bone  by  anchylosis. 

When  the  tooth  is  auchylosed  by  its  outer  side  to  an 
external  parapet  of  bone,  the  creature  is  said  to  be  "  pleu- 
rodont,"  when  by  the  end  of  its  base  it  is  attached  to  the 
summit  of  a  parapet  it  is  "  aci'odont." 

The  succession  of  teeth  in  the  Lizards  is  constant,  new 
teeth  being  developed  at  the  inner  side  of  the  bases  of  the 
old  teeth,  which  become  undermined  by  absorption  and  fall 
off  when  the  successioual  tooth  has  attained  to  a  certain 
stage  in  its  development. 

The  accompanying  figure  of  the  lower  jaw  of  a  Monitor 


242  A    MANUAL    OF   DENTAL    ANATOMY. 

lizard  will  give  an  idea  of  a  dentition  common  in  the- 
group.  The  teeth  are  not  very  large  nor  very  numerous-, 
there  being  about  30  in  the  jaw ;  towards  the  front  of  the 
mouth  they  are  a  little  more  pointed  than  at  the  back,  but 
the  differences  in  this  respect  are  not  striking. 

At  the  inner  side  of  the  bases  of  the  teeth   are   seen 


Fig.  101  (1). 


r'\ 


\ 


foramina  which  lead  into  the  spaces  in  which  new  teeth  are 
being  developed. 

Amongst  the  lizards  considerable  vai'iety  in  the  form  of 
the  teeth  themselves  exists,  some  having  thin  serrated 
edges,  others  being  exceedingly  blunt  and  rounded,  but  in 
the  general  disposition  of  the  teeth  there  is  considerable 
uniformity. 

The  teeth  of  some  lizards  consist  at  their  apices  of  ordinary 
bard  dentine,  with  a  simple  central  pulp  cavity,  but  at 
their  bases  of  plicidentiue  with  numerous  svibdivisions  of 
the  pulp  cavity,  as  is  seen  in  the  Monitor  lizards  (Varanus, 
see  p.  77).  One  Mexican  lizard  (Helodermus),  has  the  re- 
putation of  being  poisonous,  and  has  teeth  which  are 
grooved  both  back  and  front ;  but  it  is  doubtful  whether  its 
harmful  powers  have  not  been  exaggerated. 

(^)  Lower  jaw  of  a  Lizard  (Varanus  GouMiij.  a.  Foramina  leading  to 
cavities  of  reserve. 


THE    TEETH    OF   REPTILES.  243 

Vaso-dentiue  also  occurs  in  the  teeth  of  some  sauriaus,  as 
for  example,  in  those  of  the  gi'eat  extinct  Iguanodon,  in 
which  it,  roughly  speaking,  formed  the  inner  half  of  the 
crown,  the  outer  moiety  consisting  of  hard  dentine.  In 
addition  to  this  peculiarity,  the  teeth  of  Iguanodon  were 
remarkable  for  the  partial  distribution  of  the  enamel,  which 
was  strongly  ridged,  the  ridges  being  seiTated,  and  was  con- 
fined to  the  outer  side  of  the  crown.  Thus  at  the  outside 
came  the  hardest  tissvie,  the  enamel;  next  the  harder  dentine 
and  on  the  inside,  the  softer  vaso-dentine.  Hence,  as  the 
tooth  woi'e  down,  a  sharp  edge  was  long  preserved. 

There  is  a  New  Zealand  lizard,  to  which  the  several  names 
of  Hatteria,  Sphenodon,  and  Rhyncocephalus  have  been  given, 
which  has  a  very  peculiar  dental  armature  (Dr.  Giiiither, 
Phil.  Trans.,  1867). 

The  inter-maxillary  bones  are  armed  with  two  teeth,  so 
large  as  to  be  co-extensive  with  the  whole  bone  in  width, 
and  of  a  form  which  recalls  that  of  the  gnawing  incisors 
of  Rodents ;  the  other  teeth  are  quite  small,  and  "acrodont " 
in  their  attachment. 

But  the  great  peculiarity  of  Hatteria  is  that  the  alveolar 
margins  of  the  jaws  are  sharp,  and  when  the  teeth  are  worn 
down,  which  would  happen  in  adult  specimens,  the  actual 
sharp  margins  of  the  bone  come  into  play  as  masticatory 
organs,  near  to  the  front  of  the  mouth.  It  occurred  to  me 
as  probable  that  the  surface  thus  exposed  might  be  coated 
with  dentine,  but  a  microscopic  examination  of  one  of  the 
specimens  in  the  British  Museum,  which  I  was,  by  the 
kindness  of  Dr.  Gllnther,  enabled  to  make,  proved  that  tlie 
dense  ivory-like  surface  which  serves  the  purposes  of  masti- 
cation is  true  bone,  and  has  no  relation  to  dental  structure. 

There  are  very  few  other  instances  of  actual  bone,  un- 
coated  by  dental  tissues,  being  used  for  masticatory  pui'poses. 

The  great  extinct  Dicynodon,  an  African  fossil,  also 
had     sharp   trenchant     margins    to    its    jaws ;    it    is    not 

R  2 


244  A    MANUAL    OF   DENTAL    ANATOMY. 

known  whether  these  were  sheathed  in  horny  cases  like 
those  of  the  turtles,  or  whether  the  bones  thcmseh'es  came 
into  use,  as  in  Hatteria.  But  the  most  striking  peculiarity 
of  Dicynodon  was  the  co-existence  with  such  jaws  of  a  pair 
of  very  large  caniniform  tusks,  extending  downwards  and 
forwards  from  the  upper  jaw,  and  growing  from  persistent 
pulps,  a  thing  altogether  exceptional  in  the  reptilian  class. 

The  dentition  of  Ophidian  reptiles  (snakes)  is  very  uni- 
form; they  may  be  conveniently  divided  into  two  groups,  the 
poisonous  and  the  non-venomous  snakes. 

Non-venomous  snakes  have  one  row  of  teeth  in  the  lower 
jaw,  and  two  rows  in  the  upper  jaw  ;  in  the  latter  the 
maxillary  bones  carry  one  row,  while  a  parallel  internal  row 
is  supported  upon  the  palatine  and  pterygoid  bones. 

The  teeth  are  in  both  groups  strongly  recurved,  and  are 
firmly  anchylosed  to  the  bone  ;  they  consist  of  a  central 
body  of  vmvascular  dentine,  coated  by  a  very  thin  layer  of 
enamel  (there  is  not,  as  is  generally  supposed,  any  layer  of 
cementum,  the  enamel  having  been  erroneously  supposed  to 
be  such). 

The  two  halves  of  the  lower  jaw  are  connected  at  the 
symphysis  by  an  exceedingly  elastic  ligament ;  their  articu- 
lation with  the  base  of  the  skull  through  the  medium  of  an 
elongated  movable  quadrate  bone,  is  also  such  as  to  allow 
of  their  being  widely  separated  from  the  skull  and  from  one 
another,  which  allows  of  the  dilatation  rendered  necessary 
by  the  large  size  of  the  creatures  which  a  snake  swallows 
whole. 

The  teeth  of  the  snake  are  simply  available  for  seizing 
prey  and  retaining  it,  as  the  snakes  invariably  swallow  their 
prey  whole,  and  in  no  sense  masticate  it. 

As  the  object  to  be  swallowed  is  often  so  disproportionately 
large  as  to  make  the  process  of  deglutition  appear  an  im- 
possibility, the  mouth  and  pharynx  have  to  undergo  great 
dilation.     Tlie  arrangements  which  combine  to  give  to  the 


THE    TEETH    OF   REPTILES. 


lower  jaw  its  mobility  have  just  been  alluded  to ;  the 
successional  tooth  germs,  which  are  very  numerous,  are 
also  arranged  in  the  snake  in  an  unusual  position,  which  by 


bringing  them  very  close  to  the  surface  of  the  bone,  to 
which  they  lie  parallel,  renders  them  less  liable  to  displace- 
ment and  injury  than  they  would  have  been  had  they  been 
placed  vertically,  as  they  are  in  all  other  creatures  ;  while 
in  addition  to  the  advantage  of  protection  by  position,  they 
are  wrapped  i-ound  by  a  sort  of  adventitious  capsule  of  con- 
nective tissue. 


(^)  Developing  teeth  of  a  Snake.  /.  Oral  epithelium,  e.  Neck  of  the 
enamel  organs,  h.  Dentine  pulp.  c.  Enamel  cells,  d.  Dentine.  1,  2. 
Very  young  germs.     3,  4.  Older  germs. 


246  A    MANUAL    OF  DENTAL   ANATOMY. 

As  the  teeth  during  their  development  are  thus  lying  down 
parallel  with  the  length  of  the  jaw-bone,  when  the  period 
for  their  replacing  a  predecessor  arrives,  they  have  not  only 
to  move  \ipwards,  but  also  to  become  erected  ;  how  this 
is  done  remains  a  mystery,  for  I  have  been  quite  unable  to 
discern  the  means  by  w^hich  it  is  accomplished. 

When  a  snake  has  seized  its  food,  which  it  retains  by 
means  of  its  many  sharp  recurved  teeth,  it  slowly  swallows 

Fig.  103  ('). 


it  by  advancing  first  its  lower,  then  its  upper  jaw,  till  it 
thus,  so  to  speak,  forces  itself  over  the  body  of  its  prey. 
When  this  latter  is  large,   deglutition  is  a   very  lengthy 

(^)  One  half  of  the  skull  of  a  Python  (without  the  lower  y<.m)  seen  from 
below,  a.  Intermaxillary  bone.  h.  Maxillary  bone,  carrying  the  outer 
row  of  teeth,  c.  d.  Palatine  bone  and  pterygoid  bone,  the  teeth  upon 
which  constitute  the  inner  or  second  row  of  teeth. 


THE    TEETH    OF   REPTILES.  247 

process,  but  an  English  snake  can  swallow  a  moderate-sized 
frog  with  considerable  rapidity. 

There  is  an  African  snake  (Rachiodon)  which  has  none  but 
rudimentary  teeth;  its  food  consists  of  eggs,  which  thus 
escape  breakage  until  they  reach  the  oesophagus,  into  which 
spinous  processes  from  the  under  surface  of  the  vertebrse 
project,  and  there  serve  to  break  the  egg;  snakes  with 
their  dentitions  similarly  modiiied  exist  also  in  India  (e.g., 
Elachistodon). 

It  has  already  been  mentioned  that  the  non-venomous 

Fig.  104  (i). 


have  two  complete  rows  of  teeth  in  the  upper  jaw, 
the  outer  row  being  situated  on  the  maxillary  bones,  the 
inner  upon  the  palatine  and  pterygoid  bones.  The  teeth  of 
such  snakes  as  the  Pythons  are  all  simple  recurved  cones, 
and   ai-e   none   of  them   either   o-rooved  or   canaliculated."* 


(')  Head  and  jaws  of  Hydropliis.     The  maxillary  bone  (6),  instead  of 

carrying  a  complete  series  of  teeth,  is  armed  with  a  few  teeth  only  near  to 

the  front.     The  foremost  tooth  is  canaliculated,  and  forms  the  poison  fang. 

('-)  It  has  been  proposed  to  divide  the  Ophidia  into  groups,  distinguished 

by  the  presence  or  absence  of  gi-ooved  teeth,  thus  : — 

i.  Aglyphodontia.     No  grooved  or  canaliculated  maxillary  teeth, 
ii.    Opisthof/lyphia.     Some  of  the  posterior  maxillary  teeth  grooved, 
iii.   Protcroylyphki,     Anterior  maxillary  teeth  grooved. 

Posterior  maxillary  teeth  solid. 
iv.  Solenpglyphia.      Maxillary  teeth  few,   canaliculated — poisonous 


248  A    MANUAL    OF   DENTAL    ANATOMY. 


Some  of  the  harmless  snakes,  however,  have  particular- 
teeth  which  are  developed  to  a  greater  length  than  the  rest^ 
and  others  have  the  posterior  teeth  on  the  maxillary  bones 
grooved ;  but  the  statement  that  this  grooving  serves  to 
convey  an  acrid  saliva  into  the  wound  inflicted  rests  on  in- 
sufficient foundation.  The  poisonous  snakes  are  charac- 
terized by  a  shortening  of  the  series  of  teeth  carried  upon 
maxillary  bone,  and  by  the  front  tooth  of  the  series  being 
developed  to  much  greater  length  than  those  which  lie 
behind  it.  Thus  Hydrophis,  a  genus  of  poisonous  sea-snakes, 
has  five  or  more  teeth  upon  the  maxillary  bone,  the  fore- 
most of  which  is  much  the  largest,  and  this  largest  tooth  is 
so  deeply  grooved  upon  its  anterior  surface  as  to  be  converted 
into  a  tube,  the  tube  serving  to  convey  the  poison  into  the 
wounds  inflicted  by  it. 

Poisonous  snakes  which  have  several  teeth  upon  the 
maxillary  bone  for  the  most  part  present  some  little  external 
resemblance  to  the  harmless  snakes,  and  are  called  "colu- 
brine  poisonous  snakes  "  (coluber  being  the  name  of  a  genus 
of  harmless  snakes)  ;  they  present  transitional  characters 
between  these  and  the  more  specialised  or  "viperine"  poison- 
ous snakes.  The  Cobra  is  a  familiar  example  of  a  colubrine 
poisonous  snake,  and  almost  all  the  venomous  snakes  of 
Australia  belong  to  this  group.  Their  poison  fangs  are  not 
very  long,  and  they  remain  constantly  erect,  being  anchy- 
losed  to  the  bone  (the  maxilla)  which  is  long  and  not  move- 
able, and  which  also  carries  a  varying  number  of  small 
insignificant  teeth  behind  the  poison  fang. 

In  the  viperine  poisonous  snakes  (PufF-Addei',  Ptattle- 
snake,  Vipers,  &c.,)  the  poison  apparatus  is  yet  more 
specialised.  The  maxillary  bone  carries  no  teeth  at  all 
behind  the  poison  fang  ;  it  is  so  reduced  in  length  as  to  be 
of  squarish  form,  and  is  so  articulated  to  the  skull  as  to  be 
movable. 

The  poison  fang  is  of  great  length,  so  that  if  constantly 


THE    TEETH    OF  REPTILES. 


249 


erect  it  would  be  much  in  the  way ;  -when  it  is  out  of  use, 
however,  it  is  laid  flat  aloug  the  roof  of  the  mouth,  and  is 
only  erected  for  the  purpose  of  striking ;  when  in  repose  it 
is  altogether  hidden  by  a  fold  of  mucous  membrane,  which, 
when  it  is  erected,  becomes  tightly  stretched  over  a  part  of 
its  anterior  surface,  and  serves  to  direct  the  poison  do\vn 
the  poison  canal  by,  to  a  great  extent,  preventing  its  escape 
around  the  exterior  of  the  tooth. 

The  mechanism  by  which  the  poison  fang  is  erected  is 
thus   described   by   Professor   Huxley  (Anatomy  of  Verte- 


mi. 


brated  Animals,  p.  241)  : — "When  the  mouth  is  shut  the 
axis  of  the  quadrate  bone  is  inclined  downwards  and  back- 
wards. The  pterygoid,  thrown  back  as  far  as  it  can  go, 
straightens  the  pterygo-palatine  joint,  and  causes  the  axis 
of  the  palatine  and  pterygoid  bones  to  coincide.  The  trans- 
verse, also  earned  back  by  the  pterygoid,  similarly  pulls  the 
posterior  part  of  the  maxilla  and  causes  its  proper  palatine 
face,  to  which  the  great  channeled  poison  fangs  are  attached, 


(')  Side  and  front  view  of  tlie  skull  of  Craspedocepkalus  mdas.  A 
bristle  is  passed  down  the  poison  canal.  3fx.  Maxillary  bones.  3fn. 
Mandible.  PL  Palatine  bones.  Pt.  Pterygoid  bones.  Qu.  Quadrate 
bone.     T.  Transverse  bone. 

A.     Side  view.  B.     Front  view. 


250  A    MANUAL    OF   DENTAL    ANATOMY. 

to  look  backwards.  Hence  these  fixngs  lie  along  the  roof  of 
the  mouth,  concealed  between  folds  of  the  mucous  mem- 
brane. But  when  the  animal  opens  its  mouth  for  the 
purpose  of  striking  its  prey,  the  digastric  muscles,  pulling 
up  the  angle  of  the  mandible,  at  the  same  time  thrust  the 
distal  end  of  the  quadrate  bone  forwards.  This  necessitates 
the  pushing  forward  of  the  pterygoid,  the  result  of  which 
is  twofold  :  firstly,  the  bending  of  the  pterygo-palatine  joint; 
secondly,  the  partial  I'Otation  of  the  maxillary  upon  its 
lachrymal  joint,  the  hidden  edge  of  the  maxillary  being 
thrust  downwards  and  forwards. 

"In  virtue  of  this  rotation  of  the  maxillary  through  about 
a  quarter  of  a  circle,  the  dentigerous  face  of  the  maxilla 
looks  downwards  and  the  fangs  are  erected  into  a  vertical 
position.  The  snake  '  strikes  '  by  the  simultaneous  contrac- 
tion of  the  crotaphite  muscle,  part  of  which  extends  over 
the  poison  gland,  the  poison  is  injected  into  the  wound 
through  the  canal  of  -the  fang,  and  this  being  withdrawn, 
the  mouth  is  shut,  all  the  previous  movements  reversed, 
and  the  parts  return  to  their  first  position." 

The  poison  fang  is  a  long,  pointed,  slightly  recm-ved  tooth, 
traversed  by  a  canal  which  commences  on  its  front  surface, 
near  to  the  bone,  and  terminates  also  on  its  front  surface,  a 
little  distance  short  of  its  point ;  in  the  figure  a  bristle  has 
been  passed  through  it,  and  shows  the  points  where  it  com- 
mences and  terminates.  This  tube  conveys  the  poison  into 
the  puncture,  its  upper  orifice  being  in  close  relation  with 
the  end  of  the  duct  of  the  poison  gland. 

It  has  been  mentioned  that  some  snakes  which  have  not 
definite  poison  fimgs  have  a  few  of  the  large  posterior  teeth 
grooved  upon  their  front  surfaces,  the  object  of  this  grooving 
being,  as  a  matter  of  conjecture,  to  convey  a  more  or  less 
poisonous  saliva  into  the  w^ounds  inflicted  by  them. 

By  imagining  such  an  anterior  groove  to  be  deepened, 
and  finally  converted  into  a  canal  by  its  edges  growing  up 


THE    TEETH    OF   REPTILES. 


251 


and  meeting  over  it,  we  shall  have  a  fair  conception  of  the 
nature  of  the  tube  in  a  poison  fang,  which  is  thus  really 
outside  the  tooth ;  which  might  thus,  as  least  in  its  canali- 
culated  part,  be  regarded  as  a  thin  flattened  tooth  bent  round 
so  as  to  form  a  tube.  Just  as  there  are  gradations  in  the 
armature  of  the  maxillary  bone,  which  link  together  the 
extreme  form  of  the  harmless  Python,  and  the  venomous 
Rattlesnake,  so  there  are  gradations  in  the  form  of  the 
poison  tooth,  in  the  degree  in  which  the  groove  is  converted 
into  a  canal. 

In  colubriue  poisonous  snakes  the  canal  is  visible  on  the 


Fig.  106  ('). 


exterior  of  the  tooth,  where  an  apparent  fissure  marks  the 
point  where  the  two  lips  of  the  groove  have  met.  Thus  the 
poison  fang  of  Hydrophis,  although  in  a  part  of  its  length 


(')  Transverse  section  of  tooth-sac  of  poison  fang  of  Viper,  prior  to  the 
complete  closure  of  the  poison  tube  by  the  meeting  together  of  the  two 
cornua  of  the  dentine. 


252  A    MANUAL    OF   DENTAL    ANATOMY. 

the  canal  is  quite  closed  in,  has  a  very  marked  line  along 
its  front,  and  in  section  it  looks  much  as  would  the  dentine 
in  Fig.  106,  if  the  two  cornuahad  their  rounded  extremities 
brought  together  into  actual  contact,  without,  however, 
their  rounded  outline  being  altered. 

But  in  the  poison  fang  of  a  viperine  snake  the  lips  of  the 
groove  are  flattened  and  fitted  to  one  another,  so  that  not  a 
vestige  of  the  join  can  be  seen  upon  the  smooth  exterior  of 
the  tooth.  In  the  accompanying  figure  the  pulp  cavity  is 
seen  to  be  a  thin  flattened  chamber  partly  surrounding  the 
tube  formed  for  the  conveyance  of  the  poison. 

Fig.  107  ('). 


ex 


The  poison-fang  is  exceedingly  sharp,  its  point  being  con- 
tinued some  little  distance  beyond  the  place  where  the 
poison  canal  opens  on  the  front  of  the  tooth ;  this  disposi- 
tion of  parts  has  been  copied  in  the  points  of  syringes  for 
making  subcutaneous  injections. 

(')  Transverse  section  of  the  poison  fang  of  a  Eattlesnake.  ".  Pulp- 
cavity,     d.   Dentine. 


THE    TEETH    OF   REPTILES.  253 

The  dentine  is  continued  down  to  a  very  fine  point,  and 
it  is  cased  by  an  exceedingly  tliin  layer  of  enamel,  not 
much  more  than  -gi^  of  an  inch  in  thickness  in  our  common 
English  viper :  thus  the  utmost  sharpness  is  secured, 
without  loss  of  elasticity,  which  would  have  ensued  had 
its  point  been  made  up  of  brittle  enamel  only.  Enamel 
covers  the  whole  exterior  of  the  tooth  but  does  not  extend 
into  the  poison  canal  in  the  viperine  snakes  ;  in  Hydrophis 
I  believe  that  it  does.  As  the  point  is  simple,  the  tooth 
germ  of  a  poison-fang  only  becomes  distinguishable  from 
that  of  another  ophidian  tooth  after  the  tip  of  the  tooth  has 
been  formed,  when  a  groove  appears  in  its  side  (see  8  and  9, 
in  Fig.  108). 

It  being  the  habit  of  poisonous  snakes  to  make  use  of 
these  weapons  to  kill  their  prey,  which  they  consequently 
do  not  swallow  alive,  it  would  obviously  subject  them  to 
no  little  inconvenience  to  be  without  these  weapons  for 
any  considerable  length  of  time,  while  from  their  habit  of 
striking  living  prey  the  long  fangs  must  be  very  liable  to 
being  broken  off  by  the  jumping  away  of  the  creature  struck, 
to  say  nothing  of  the  great  force  with  which  the  blow  is 
given. 

In  the  most  typical  (viperine)  poisonous  snakes  the 
succession  of  teeth  is  conducted  upon  a  plan  which  is 
unique,  and  which  is  excellently  adapted  to  save  loss  of 
time  in  the  replacement  of  a  lost  poison  fang.  Upon  the 
movable  maxillary  bones  there  is  space  enough  for  two 
poison  fangs,  side  by  side ;  only  one,  however,  is  fully 
anchylosed  to  the  bone  at  a  time,  and  occupies  a  place  to 
the  extreme  right  or  extreme  left  of  the  bone,  leaving  vacant 
space  for  another  by  its  side. 

When  the  tooth  in  use  falls,  it  will  be  succeeded  by  a 
tooth  upon  the  vacant  spot  by  its  side,  not  upon  the  spot 
upon  which  itself  stood,  so  that  the  places  on  the  right 
and  the  left  of  the  bone  are  occupied  alternately  by  the  tooth 


254  A    MANUAL    OF   DENTAL    ANATOMY. 

in  use.  Thus,  in  Fig.' 105,  the  poison  [fang  of  the  snake's 
right  side  is  seen  occupying  a  position  on  the  extreme  out- 
side of  the  maxillary  bone,  while  its  left  poison  fang  is  fixed 
on  the  inside  of  the  maxillary  bone. 

The  upper  boundary  of  Fig.  108  is  formed  by  the  flap 
of  mucous  membrane  which  covers  in  the  poison  feng  when 
at  rest.     Isos.  1  and  2  lie  in  the  pouch  formed  by  it,  the 

Fia.  108  ('). 


^'    -.0 


section  happening  to  be  tal-jju  fiuiu  a  specimen  in  which  the 
tooth  was  about  to  be  changed.  In  most  specimens  one 
tooth  only,  the  tooth  actually  in  use,  is  seen  in  this  position. 
A  Hap  hanging  free  across  this  space  serves  apparently  to 

(')  Ti-ansverse  section  of  the  reserve  poison  fangs  of  a  Viper.  1.  Tootli 
at  present  in  use,  in  its  recumbent  position  ;  were  it  erect,  it  would  be 
witiidrawn  from  view,  or  else  seen  in  longitudinal  section.  2.  Tooth  which 
will  next  succeed  to  No.  1.  3,  4,  5,  &c.  Tooth -sacs  numbered  in  the  order 
in  which  thev  will  succeed. 


THE    TEETH    OF   REPTILES.  255 

keep  teeth  of  the  oue  series  from  getting  over  to  the  other 
side,  and  probably  serves  to  hold  in  place  the  reserve  tooth 
when  the  older  tooth  is  erected  for  biting. 

The  reserve  poison  fangs,  as  many  as  ten  in  nnmber  in 
the  Rattlesnake,  are  likewise  arranged  in  two  parallel  series, 
in  which  the  teeth  exist  in  pairs  of  almost  equal  age  ;  the 
tooth  in  use  is  thus  derived  alternately  from  the  one  and 
the  other  series,  as  is  indicated  by  the  consecutive  numbers 
in  the  figure,  a  septum  of  connective  tissue  keeping  the  two 
series  of  teeth  distinct  from  one  another. 

The  teeth  being  arranged  in  pairs  of  almost  equal  age, 
suggest  that  the  succession  is  both  rapid  and  regular.  All 
the  reserve  teeth  lie  recumbent  in  and  behind  the  sheath 
of  mucous  membrane  which  covers  in  the  functional  tooth. 

This  arrangement  of  the  successional  teeth  in  a  paired 
series  does  not  exist  in  the  Cobra,  in  which  the  successional 
teeth  form  but  a  single  series ;  perhaps  this  may  serve  to 
explain  the  preference  of  the  snake  charmers  for  the  Cobra, 
which  would  probably  take  longer  to  replace  a  removed 
poison  fang  than  a  viperine  snake  would. 

But  in  the  colubrine  venomous  snakes  the  successional 
poison  fang  sometimes  makes  its  way  to  a  spot  a  little  to  the 
side  of  its  predecessor,  so  that  there  may  possibly  be  no  loss 
of  time  :  and  notwithstanding  that  they  are  in  a  measure 
transitional  forms  between  the  harmless  and  the  viperine 
snakes,  some  of  them  are  most  virulently  poisonous  and 
deadly  in  their  bite  {'). 

This  arrangement  of  hro  distinct  chains  of  younger  de- 
veloping organs,  all  destined  to  keep  the  creature  always 
supplied  with  one  organ  in  a  state  of  efficiency,  is,  so  far  as 
I  know,  without  parallel. 

Like  other  ophidian  teeth  the   poison  fangs  become  an- 


(1)  I  have  given  a  more  detailed  account  of  the  suecession  of  poison  fangs 
in  the  Philos;  Trans.,  1876,  Part  i. 


256 


A    MANUAL    OF  DENTAL    ANATOMY. 


chylosed  to  the  bone  which  carries  them,  their  secure 
fixation  being  aided  by  the  base  of  the  tooth  being  fluted,  as 
well  as  by  a  sort  of  buttress  work  of  new  bone  being  thrown 
out  to  secure  each  new  poison  fang  as  it  comes  into  place. 

The  poison  is  secreted  by  a  salivary  gland  homologous 
with  the  parotid  ;  by  an  especial  arrangement  of  the  muscles 
and  fascia  about  it  the  erection  of  the  poison  fang  and  the 
infliction  of  the  bite  cause  a  copious  stream  of  poison  to  be 


Fig.  109  ('). 


ejected.  The  duct  terminates  in  a  sort  of  papilla,  close  to 
the  superior  orifice  of  the  tube  in  the  fang  :  the  passage  of 
a  considerable  portion  of  the  poison  down  the  tube  is  secured 
by  the  close  apposition  of  a  shield  of  mucous  membrane, 
which  is  strained  over  the  erected  tooth. 

In  Crocodilia  tlie  teeth  are  confined  to  the  margins  of 

(')  Jaws  of  the  Crocodile.  Tlie  first,  fourth,  and  eleventh  teeth  in  the 
lower  jaw,  and  the  third  and  ninth  in  the  upper,  arc  seen  to  attain  to  a 
larger  size  than  the  others. 


THE    TEETH    OF   REPTILES. 


the  jaws,  where  they  are  very  formidable  in  size  and  shar])- 
ness.  The  individual  teeth  are  generally  conical,  sharply 
pointed,  and  often  a  little  compressed  from  side  to  side,  so 
as  to  possess  sharp  edges  ;  but  they  vary  much  in  form  in 
different  species. 

The  teeth  are  lodged  in  distinct  tubular  alveolar  cavities, 
to  the  walls  of  which  they  do  not  become  anchylosed,  and 
they  are  tolerably  constant  in  number  in  the  same  species. 

In  parts  of  the  mouth  certain  teeth  are  developed  to  a 
greater  length  than  those  nearest  to  them ;  thus,  in  the 
Crocodile  proper,  the  first  and  fourth  lower  teeth  are  spe- 


FiG.  110  (1). 


^/ 


cially  large,  while    in    the    extinct  African   Galesaurus  the 
diflPerence  is  so  marked  that  both  in  the  upper  and  lower 


(')  Transverse  section  of  tlie  lower  jaw  of  a  yonng  Alligator,  a.  Oral 
epithelium,  h.  Bone  of  socket,  d.  Dentine  of  old  tooth.  2.  Tooth  next 
in  order  of  succession,  which  is  causing  absorption  of  one  side  of  the  base 
of  the  older  tooth.      3.   Young  tooth  a;erm. 


268  A    MANUAL    OF   DENTAL    ANATOMY. 

jaws  the  teeth  might  be  grouped  as  incisors  and  canines, 
so  far  as  size  and  probable  function  go  in  such  a  classification. 

In  structure  the  teeth  of  crocodiles  consist  of  hard,  fine 
tubed  dentine,  with  an  investing  cap  of  enamel,  and  in 
addition  a  coating  of  cementum  on  their  implanted  por- 
tions. As  already  mentioned,  they  are  implanted  in  tubu- 
lar sockets ;  new  successional  teeth  are  being  continually 
developed  at  the  inner  side  of  their  bases,  and  as  these  at- 
tain to  a  certain  size,  absorption  attacks  the  base  of  the 
older  tooth,  and  its  successor  moves  into  the  space  so  gained, 
so  that  it  comes  to  be  situated  vertically  beneath  the  older 
tooth.  In  its  further  growth  it  causes  yet  more  absorption 
of  the  older  tooth,  which  it  ultimately  pushes  out  in  front 
of  it,  sometimes  carrying  the  remains  of  the  old  tooth  like 
a  cap  upon  its  own  apex  when  it  first  emerges.  Each  new 
tooth  vertically  succeeds  its  predecessor ;  hence  no  additional 
teeth  are  added,  but  the  young  newly  hatched  crocodile  has 
as  many  teeth  as  a  full  grown  one. 

In  the  extinct  Ichthyosaurus  the  teeth,  while  forming  an 
armatu^re  not  unlike  that  of  some  of  the  crocodiles,  were 
not  implanted  in  distinct  sockets,  but  were  lodged  in  a 
continuous  shallow  groove,  with  but  slight  indications  of 
transverse  divisions. 

The  huge  Dinosauria,  some  of  which  must  have  been 
thirty  feet  in  length,  had  teeth  implanted  in  imperfect 
sockets,  the  outer  alveolar  wall  being  considerably  higher  than 
the  inner,  and  the  transverse  septa  not  very  complete.  The 
I'oots  of  the  teeth  were  more  or  less  perfectly  cylindrical, 
and  the  enamelled  crowns  compressed  and  expanded,  with 
trenchant  edges.  The  tooth  of  the  Iguanodon  will  sen-e  as  a 
fair  example  of  a  Dinosaurian  tooth  :  the  crown  is  greatly  ex- 
panded, and  presents  anterior  and  posterior  sharp  notched 
margins  ;  the  enamel  is  laid  over  the  ou^ter  surface  of  upper 
teeth,  and  the  inner  of  lower  teeth.  The  enamelled  surface 
is  ridged,  so  that- as  it  wears  down  a  notched  edge  is  main- 


THE    TEETH    OF   REPTILES.  259 

tained.  Moreover  the  maintenance  of  a  sharp  edge  is  further 
secured  by  the  dentine  on  the  enamelled  side  of  the  crown 
being  of  the  hard  unvasciilar  variety,  that  on  the  inner  being 
vaso-dentine  and  therefore  softer.  The  remnant  of  the  pnlp 
ossifies,  and  comes  into  use,  as  these  teeth  remained  at  work 
until  worn  quite  to  a  flat  surface.  The  root  portion  was 
smooth,  round,  and  curved. 

Professor  Marsh  (American  Journal  of  Science,  March, 
1880)  has  described  and  figured  a  pecuHar  Diuosaurian  den- 
tition, in  a  reptile  to  which  he  gives  the  name  of  Stegosaurus  ; 
the  teeth  are  slightly  compressed  transversely,  and  are 
covered  with  a  thin  enamel ;  the  roots  are  long  and  slender, 
implanted  weakly  in  separate  sockets.  But  at  the  inner  side 
of  the  roots  of  the  teeth  in  use  were  no  less  than  five  suc- 
cessional  teeth,  in  graduated  stages  of  development,  ready 
to  ultimately  take  its  place  ;  so  large  a  number  of  successional 
teeth  has  uot  hitherto  been  met  with  in  a  Dinosaur. 

A  very  remarkable  carnivorous  reptile  as  large  as  a  lion 
has  been  described  by  Professor  Owen  (Quart.  Journal 
Geolog.  Society,  1876,)  under  the  name  of  Cynodraco  major 
for  the  reception  of  which  he  proposes  a  new  reptilian  order, 
that  of  Theriodontia.  Its  dentition  is  not  completely  known, 
but  it  possessed  in  the  lower  jaw  eight  incisors,  of  Avhich  the 
first  is  the  smallest,  and  a  canine  of  moderate  size.  The 
upper  incisors  are  not  known,  but  there  were  a  pair  of  upper 
cabines  of  such  size  that  they  extended  down  along  the 
outside  of  flattened  poi'tion  of  the  lower  jaw,  like  the 
canine  teeth  of  Machairodus.  The  hinder  margins  of  these 
canines  were  trenchant,  and  finely  serrated. 

The  Pterosauria,  or  flying  reptiles,  have,  since  the 
discovery  of  toothed  birds,  become  of  special  interest  to 
the  odontologist.  The  wings  were  stretched  membranes, 
like  those  of  a  bat,  and  the  measurement  across  their  tips 
in  some  of  the  largest  must  have  been  twenty-five 
feet ;  but  most   of  those  known  were    much    smaller,    from 


260  A    MANUAL    OF   DENTAL    ANATOMY. 

10  to  15  inches  iu  total  length  of  body.  In  the  Pterodactyls 
the  jaws  are  furnished  with  long,  slender,  sharp  teeth  in 
their  whole  length :  but  in  Ramphorhynchus  the  anterior 
extremities  of  the  jaws  are  without  teeth,  and  it  has  been  con- 
jectured that  tliese  portions  were  sheathed  in  horny  beaks. 

And  Prof.  Marsh  (American  Journal  of  Science,  1876,)  has 
discovered,  in  the  same  formation  in  which  he  found  the 
toothed  birds,  several  species  of  Pterodactyls  wholly  without 
teeth,  for  which  the  generic  name  Pteranodon  is  proposed. 

The  jaws,  which  are  more  like  those  of  birds  than  those  of 
any  known  reptile,  show  no  traces  of  teeth,  and  the  pre- 
maxillaries  seern  to  have  been  encased  in  a  horny  covering. 


The  Teeth  of  Biiids. 

Piior  to  the  discovery  by  Professor  Marsh  of  Yale  College, 
in  1870,  of  the  remains  of  birds  with  teeth  in  the  cretaceous 
formations  of  Western  Kansas,  little  was  with  certainty 
known  about  the  existence  of  teeth  in  any  bird,  although 
one  or  two  fossils,  leading  to  the  suspicion  that  birds  might 
have  possessed  teeth,  were  known.  The  state  of  knowledge 
up  to  that  time  has  been  clearly  summarised  by  Mr.  Wood- 
ward (Popular  Science  Review,  1875,)  to  this  effect :  that 
it  had  been  long  supposed  that  no  examples  of  teeth  were  to 
be  met  with  amongst  the  birds,  although  some,  such  as  the 
Merganser,  have  the  margins  of  the  bill  serrated,  so  that  the 
functions  of  teeth  are  discharged  by  this  horny  armature  of 
the  jaws. 

It  is  noteworthy  that  the  margin  of  the  bone  of  the  jaws 
is  also  serrated,  each  serration  corresponding  to  a  similar 
serration  in  the  bill.  In  the  fossil  bird  described  by  Pro- 
fessor Owen,  from  the  London  clay,  under  the  name  of 
Odontopteryx  toliapicus,  the  form  of  the  bill  is  not  known, 
but  the  margins  of  the  jaws  are  furnished  with  strong  bony 
prominences,    far    more    conspicuous   than    those    of    the 


THE    TEETH    OF   BIRDS.  261 

Merganser.  And  GeofFroy  St.  Hilaire  had  described  a  series 
of  vascular  pulps  as  existing  on  the  margin  of  the  jaw  of 
parroquets  just  about  to  be  hatched,  which,  though  destined 
to  form  a  horny  bill,  and  not  to  be  calcified  into  teeth,  yet 
strikingly  recalled  dental  pulps.  Then  there  is  also  the 
famous  fossil  Ai'chseopteryx,  an  anomalous  oolitic  bird,  with 
a  long  and  jointed  tail,  which  is  by  many  zoologists  believed 
to  have  possessed  teeth.  There  is  a  flaw  in  the  evidence, 
however,  inasmuch  as  the  toothed  jaw  is  not  in  situ,  and 
therefore  may  possibly  have  belonged  to  some  other  animal 
than  that  perpetuated  in  the  rest  of  the  fossil  impression, 
though  i:)robability  is  altogether  in  favour  of  its  really 
belonging  to  the  Archseopteryx. 

In  successive  expeditions,  conducted  under  great  ditiiculties 
owing  to  the  extremes  of  heat  and  cold,  and  to  the  hostility 
of  the  Indians,  the  remains  of  no  less  than  one  hundred 
and  fifty  diflFerent  individuals  referable  to  the  sub-class 
Odontornithes  have  been  obtained  by  Prof.  Marsh  ;  they 
are  classified  under  nine  genera,  and  twenty  species. 

They  are  referable  to  two  widely  different  types,  one 
group  consisting  of  comparatively  small  birds,  with  great 
power  of  flight,  and  having  their  teeth  implanted  in  distinct 
sockets  (Odontotoruse,  illustrated  by  the  genus  Ichthyornis 
as  a  type)  ;  the  other  group  consisting  of  very  large  swimming- 
birds,  without  wings,  and  having  teeth  in  grooves  (Odontolca;, 
type  genus  Hesperornis). 

In  Ichthyornis  the  teeth  were  about  twenty-one  in  num-ber 
in  each  ramus,  all  sharp  and  pointed,  and  recurved ;  the 
crowns  were  coated  with  enamel,  and  the  front  and  back 
edges  sharp  but  not  serrated. 

They  are  implanted  in  distinct  though  shallow  sockets, 
and  the  maxillary  teeth  are  a  little  larger  than  those 
opposing  them  ;  the  premaxillaries  were  probably  edentulous, 
and  perhaps  covered  with  a  horny  bill. 

In  the  lower  jaw  the  largest  teeth  occur  about  the  middle 


A    MANUAL    OF   DENTAL    ANATOMY. 


of  the  I'iimus,  those  at  its  posteiior  end  being  materially 
smaller ;  and  the  sockets  are  deeper 
and  stronger  than  in  the  upper  jaw. 
The  succession  takes  place  vertically, 
as  in  Crocodiles  and  Dinosaurs. 

The  genus  Hesperoruis,  probably 
diving  birds,  includes  species  6  feet  in 
length  :  as  has  already  been  mentioned 
the  teeth  are  not  implanted  in  distinct 
sockets,  but  lie  in  a  continuous  groove 
like  those  of  Ichthyosaurus  ;  slight  pro- 
jections from  the  lateral  walls  indicate 
a  partitioning  off  into  sockets,  but 
nothing  more  than  this  is  attained,  and 
after  the  perishing  of  the  soft  parts  the 
teeth  were  easily  displaced,  and  had 
often  fallen  out  of  the  jaws.  The  pre- 
maxillary  is  edentulous,  but  the  teeth 
extend  (piite  to  the  anterior  extremity 
of  the  lower  jaw  :  in  one  specimen 
there  arc  fourteen  sockets  in  the  max- 
illary bone,  and  thirty-three  in  the 
corresponding  lower  ramus. 

The  successional  tooth  germs  were 
formed  at  the  side  of  the  base  of  the 
old  ones,  and  causing  absorption  of  the 
old  roots,  migrated  into  the  excavations 
.,  grew  large,  and  ultimately  expelled  their  prede- 
cessors, as  is  seen  in  the  accompanying  figure. 

In  structure  these  teeth  consist  of  hard  dentine,  invested 
with  a  rather  thin  layer  of  enamel,  and  having  a  large  axial  pulp 
cavity.  The  basal  portion  of  the  roots  consists  of  osteodentine. 


so  formed, 


Q)  Mandible  of  Ichfcliyornis  (after  Prof.  Marsh).     A.  Side  view,  show- 
ing the  teeth  in  situ.     B.  View  of  upper  surface,  showing  the  sockets  in 

which  tlie  teeth  were  implanted. 


THE    TEETH    OF  BIRDS. 


The  outer  side  of  the  crown  is  nearly  flat,  the  inner  strongly 
convex  :  the  junction  of  these  surfaces  is  marked  by  a  sharp 
ridge,  not  serrated. 

In  form  the  teeth  of  Hesperornis  present  a  close  resem- 
blance to  those  of  Mosasaiu'us,  a  great  extinct  lizard. 

Indeed,    as   Prof.    Marsh    observes,    "in   all   their   main 

Fig.  1]2(1). 


features  the  teeth  of  Hesperornis  are  essentially  reptilian, 
and  no  anatomist  v/ould  hesitate  to  refer  them  to  that  class, 
had  they  been  found  alone.  Combined  with  the  other 
reptilian  characters  of  Hesperornis  ....  they  clearly 
indicate  a  genetic  connection  with  that  group." 

In  the  dentine  contour  lines  are  abundant ;  the  enamel 
is  so  dense  as  to  appear  structureless,  and  there  is  no  coronal 
cementum. 

The  foregoing  account  is  condensed  from  the  magnificent 
volume  published  by  the  United  States  Government  Geo- 
logical Exploration.  (Odontornithes,  a  monograph,  ikc,  by 
O.  C.  Marsh,  Prof,  of  Palaeontology,  Yale  College.) 

With  these  notable  exceptions,  the  jaws  of  all  known  birds 
are  toothless,  the  horny  cases  forming  their  beaks  taking  the 
places  and  fulfilling  the  functions  of  teeth. 

(0  (After  Prof  Marsh.)  A.  Hesperornis  regalis,  with  successional  tooth 
in  an  excavation  at  its  base  ;  enlarged  eight  diameters.  B.  Tooth  of  Mos- 
isauriis  princeiJS,  half  natural  size. 


CHAPTER    VIII. 

THE    TEETH    OP    MAMMALS. 

The  class  Mammalia  is  divided  into  three  groups : — 

I.   Ornlthodclphla. 

Animals  with  a  common  genito-ui-inaiy  chamber,  and  separate 
coracoid  bones  ;  no  vagina  ;  no  teats  ;  comprises  a  single  order, 
Monntrriinita,  which  contains  only  two  genera,  the  Ornithorhynchus 
and  the  Echidna. 

II.  Didrlphia. 

Animals  with  a  vagina,  &c.  ;  of  which  the  young  are  bom  in  an 
exceedingly  early  condition,  probably  without  the  formation  of  any 
placenta,  and  are  transferred  to  the  nipple  of  the  mother,  where, 
in  almost  all,  they  are  protected  by  a  fold  of  the  abdominal  integu- 
ment, which  forms  the  marsupium,  or  pouch  ;  comprises  the  single 
order  J/(cr.'<iij)ialia,  animals  now  most  largely  represented  in  Aus- 
tralia and  its  zoological  region  ;  some  few  exist  also  in  America. 

The  kangaroos,  wombats,  opossums,  &c.,  are  familiar  examples 
of  Marsupials. 

III.  I\I(infl(h'lphia. 

Placental  mammals  :  i.e.,  animals  in  which  the  foetus  acquii-es  a 
connection  with  the  parent  through  the  medium  of  a  vascular  pla- 
centa, by  means  of  which  it  is  nourished  for  a  long  time,  and  is 
ultimately  born  in  an  advanced  condition. 

The  relations  which  the  different  orders  of  placental  Mammalia 
bear  to  one  another  are  rather  complex,  and  it  is  not  possible  to 
place  them  satisfactorily  in  a  consecutive  series,  because  many  of 
the  orders  present  affinities  with,  and  are  indeed  linked  by  transi- 
tional forms  to,  not  one,  but  several  other  orders.  Professor  Flower 
(Osteology,  page  6.)  has  arranged  them  in  the  following  tabular 
manner,  each  order  being  placed  near  to  those  to  which  it  presents 
most  resemblance. 


THE    TEETH    OF  MAMMALS. 


Relation   of 

the 

existing     Mammalian    orders 

to    on( 

another : — 

Hominina. 
Primates. 

Chiropteea. 

Suniina. 
Lemurina. 

Insectivora. 

Carnivora. 
Fissipedia.     Pinnipedia. 

Hyracoida. 

Cetacea. 

RODENTIA. 

SiRENIA. 

DiNOCEEATA  (0  Perissodactyla. 

Peoboscidea.  Ungulata. 

Suina.     Tylojnnhi. 

Ai"tiodactyla. 
Tragvllna.     Pecora. 

Edentata. 

Primates  include  man   and  the  monkeys,  the  Lemiu's  connecting 
them  with  both  the  Insectivora  and  Chiroptera. 

ChirojHera  — Bats. 

Imcctivora —  Moles,  Hedgehogs,  &c. 

Carnivora  Ji,ss'n>idla — Cats,  Dogs,  and  Bears,  kc. 

Carnivora  jiinnipcilia — Seals,  Wabus,  &c. 

Cefacra — WTialebone  \\Tiales,  Sperm  "WTiales,  Porpoises,  &c. 

Sircnia  {Ilcrhivoroiis  Cctacra) — Manatee,  Dugong,  &c. 

Cngiihita  (Hoofed  Mammals) — 

(i.)  Perissodactyla,  or  odd-toed — Horse. 

Tapir,  Rhinoceros,  &c. 
(ii.)  Artiodactyla,  or  those  with  an  even  number  of  toes — Pigs 
and  their  allies.  Camels,  Ruminants,  &c. 

Dinoccrata — Gigantic  Fossil  Mammals,  somewhat  intermediate  be- 
tween Perissodactyl  Ungulata,  and  Proboscidea. 

Hyracoidca — The  anomalous  Hyrax  (Biblical  "  Cony  ")  alone. 

P/"oJf«c<V/frt— Elephants,  extinct  Mastodons,  &c. 

Bodentia — Hares,  Rabbits,  Rats,  &c. 

Edentata — Sloths,  Armadillos,  Ant-Eaters,  &c. 


To  illustrate  the  meaning  of  the  table,  the  gap  existing  between 
the  typical  Carnivora  and  the  Cetacea  is  bridged  over  by  the  seals, 
which,  though  tnie  carnivora,  are  some  of  them  near  the  Cetacea 


266  A    MANUAL    OF  DENTAL    ANATOMY. 


in  many  particulars  ;  or,  again,  the  gap  between  the  Monkeys  and 
the  Insectivora  is  bridged  over  by  the  Lemurs,  which  are  inter- 
mediate forms. 

We  do  not  yet  know  enough  of  extinct  Mammalia  to  feel  quite 
sure  of  the  true  line  of  aflfinities  between  all  the  orders,  but  the 
foregoing  table  serves  to  give  a  more  true  idea  of  our  present  know- 
ledge than  any  arrangement  in  linear  series  can  convey.  There  is 
no  animal  to  which  we  can  point  and  say  that  we  know  its  whole 
line  of  descent ;  but  the  ancestry  of  some  of  the  Ungulata  has  been 
greatly  elucidated  of  late  years,  and  the  chain  of  progressive  modi- 
fication by  which  so  highly  specialised  a  form  as  the  Horse  has  been 
arrived  at,  starting  from  a  very  much  more  generalised  foim,  is  now 
pretty  complete. 

In  a  treatise  dealing  only  with  the  teeth,  in  which  the  orders 
must  necessarily  be  taken  in  succession,  it  will  be  convenient  to 
deviate  somewhat  from  the  natural  order  for  the  sake  of  taking 
first  those  animals  whose  dentitions  are  of  the  simplest  character. 
Thus  it  is  convenient  to  describe  in  succession  the  Edentata  and 
the  Cetacea,  which  have  little  or  nothing  to  do  with  one  another, 
because  they  alike  have  teeth  of  simpler  form  than  the  rest  of  the 
Mammalia.  But,  as  far  as  possible,  the  arrangement  indicated  in 
the  table,  which  the  student  will  do  well  to  impress  upon  his  mind, 
will  be  followed  in  these  pages. 


INTRODUCTORY    REMARKS. 

Not  many  years  ago  it  was  customary  to  explain  the 
various  facts  which  were  revealed  by  the  study  of  com- 
parative anatomy  upon  the  supposition  that  there  was  some 
sort  of  type  or  standard  organization,  and  that  all  others 
were  anived  at  by  modifications  and  departures  from  this 
type,  these  modifications  being  introduced  with  a  direct 
pvirpose  in  view,  in  order  to  fit  the  creature  to  a  special 
habit  of  life. 

Among  the  matters  which  this  "  type  "  theory  sought  to 
account  for  was  this  :  when  an  animal  possesses  some  pecu- 
liar organ,  it  is  found  on  close  examination  that  it,  however 
specialised,  is  after  all  only  something  which  allied  animals 
also  possess,  only  it  has  been  exaggerated  or  developed  in  an 
unusual  manner  and  degree ;  or,  on  the  other  hand,  that 
when  an  organ  is  wanting,  the  suppressed  organ  is  not 
absolutely  abolished,  but  is  to  be  found  stunted  and  in  a 


THE    TEETH    OF   MAMMALS.  267 

rudimentary  condition,  instead  of  in  its  ordinary  size  and 
functional  activity. 

This  is  as  true  of  teeth  as  of  any  other  organs ;  indeed 
the  study  of  odontology  reveals  many  admirable  examples 
of  the  law. 

Thus  the  tusks  of  the  boar  or  of  the  Sus  babirussa,  large 
and  peculiar  though  they  be,  are  not  new  developments,  but 
ai-e  merely  the  canine  teeth  which  in  these  species  attain  to 
unusual  dimensions.  In  the  same  way  the  enormous 
straight  tusk  of  the  Xarwal  (see  Fig.  133)  is  nothing  more 
than  an  incisor  tooth  of  one  side,  the  fellow  to  which  has 
been  checked  in  its  develoi^meut ;  but  this  is  not  missing,  for 
it  remains  throughout  the  life  of  the  animal  buried  within 
its  socket.  In  the  female  Narwal  both  of  the  teeth,  being 
rudimentary,  are  permanently  enclosed  within  the  sockets, 
and  are  of  course  not  of  the  smallest  service  to  the  animal, 
directly  or  indirectly ;  fm-thermore,  as  has  been  shown  by 
Professor  Turner,  in  young  specimens,  a  second  pair  of 
rudimentaiy  aborted  incisors  are  to  be  found,  which  in  the 
adults  have  disappeared. 

The  modem  school  of  biologists,  rejecting  this  "ai-che- 
type  "  theory  as  a  far-fetched  and  unsatisfactory  hypothesis, 
refer  these  resemblances  detected  between  dentitions  upon  the 
whole  dissimilar  to  one  another  to  a  more  intelligible  cause, 
namely,  inheritance.  Assuming,  as  the  balance  of  evidence 
compels  us  to  assume,  that  the  many  divergent  forms  which 
we  observe  have  been  derived  by  progressive  modifications 
and  difierentiations  from  fewer  ancestral  forms,  we  shall  have 
no  difficulty  in  seeing  how,  by  such  processes  as  we  full  well 
know  to  occur,  namely,  the  dwindling  of  disused  organs  and 
the  exaggerated  development  of  those  used  in  an  unusual 
degi-ee,  great  differences  may  ultimately  result. 

To  illustrate  what  is  meant  by  this  so-called  "adapti^-e 
modification,"  this  suppression  of  things  that  are  not 
needed,  and  increased  development  of  those  most  used,  we 


268  A    MANUAL    OF   DENTAL    ANATOMY. 

may  recur  to  the  dentitions  of  non-venomous  and  venomous 
snakes. 

In  these  we  saw,  in  the  non-venomous  snakes,  the  max- 
illary bones  covered  by  a  row  of  teeth  sub-equal  in  size ; 
then  in  the  '  Colubrine '  poisonous  snakes  the  front  tooth  of 
those  standing  upon  the  maxillary  bone  taking  upon  itself  a 
special  and  important  office,  namely,  the  conveyance  into  a 
wound  of  a  poisonous  saliva,  and  coincidently  with  this 
tooth  attaining  its  increased  size  and  importance,  the  teeth 
behind  it  on  the  maxillary  bone  reduced  both  in  number 
and  in  size.  Going  a  step  further,  to  the  Viperine  poisonous 
snakes,  the  now  useless  small  maxillary  teeth  have  all  disap- 
jjeared,  leaving  the  poison  fang  alone,  and  of  vastly  increased 
dimensions,  to  occupy  the  whole  bone. 

But  in  many  poisonous  colubrine  snakes  three  or  four 
small  and  useless  teeth  lingering  upon  the  maxillary  bone, 
though  their  function  was  gone,  seemed  to  indicate  to  us 
in  some  measure  the  gradual  process  by  which  that  singu- 
larly perfect  adaptation  of  means  to  an  end,  the  poison 
apparatus  of  the  viper  was  arrived  at. 

It  would  be  impossible  in  these  pages  to  go  through  the 
arguments  by  wdiich  Mr.  Darwin  has  established  his  main 
propositions ;  it  must  suffice  to  say  here,  that  he  has  fully 
convinced  all  those  who  are  not  in  the  habit,  from  the  fixity 
of  early  impressions,  of  putting  many  matters  upon  another 
footing  than  that  established  by  the  exercise  of  i"eason,  that 
any  modification  in  the  structure  of  a  plant  or  an  animal, 
which  is  of  benefit  to  its  possessor,  is  capable,  nay,  is  sure 
of  being  transmitted  and  intensified  in  successive  genera- 
tions, until  great  and  material  differences  have  more  or  less 
masked  the  resemblance  to  the  parent  form. 

Just  as  man,  by  favouring  the  breeding  of  those  modifica- 
tions of  form,  &c.,  that  please  him  best,  has  been  able,  in 
the  course  of  a  few  years — in  a  length  of  time  altogether 
infinitesimal,  as  compared  with  the  time  dui'ing  which  the 


TEE    TEETH    OF  MAMMALS.  269 

surface  of  land  and  sea  has  been  of  pretty  nearly  its  present 
form,  to  say  nothing  of  the  enormously  longer  earlier  geo- 
logical epochs — to  profoundly  modify  the  breeds  of  dogs,  of 
horses,  of  numbers  of  plants,  all  of  which  are  absolutely 
known  to  have  had  a  common  origin,  so  in  nature  forces  are 
and  ever  have  been  in  perpetual  operation,  which  effect  the 
same  thing. 

A  pigeon  fancier  wants  a  pigeon  of  particular  plumage, 
with  a  few  feathers  a  little  different  from  any  pigeon  he  has 
ever  seen  or  heard  of ;(')  he  knows  by  experience  that  little 
variations  are  for  ever  arising,  and  that  by  watching  a 
sufficient  number  of  young  ones,  and  rigorously  picking  out 
those  which  at  all  tend  in  the  direction  of  what  he  wants, 
he  will  get  what  he  wants,  and  will  even  tell  you  with  confi- 
dence that  in  so  many  years  he  will  make  a  breed  with  the 
peciiliarity  desired.  And  exactly  as  the  plumage  that  was 
wanted  is  got,  so  in  nature  the  tooth  that  is  "  wanted," 
i.e.,  the  dentition  that  is  excellently  well  adapted  to  do  its 
work  is  manufactured  by  the  operations  of  that  law  known 
as  "  survival  of  the  fittest." 

It  is  quite  enough  that  one  of  the  small  variations  for 
ever  arising  in  animals  shall  be  of  advantage  to  it,  for  us  to 
see  that  the  peculiarity  is  likely  to  be  transmitted  and 
intensified  in  successive  generations. 

The  question  has  been  w^ell  presented  by  Mr.  Wallace, 
who  points  out  that  wc  must  not  think  so  much  of  varia- 
tions in  individuals  as  in  groups  of  individuals  :  for  instance, 
it  is  a  familiar  fact  that  people  vary  in  height,  so  that  any 
hundred  persons  may  be  divided  into  fifty  taller  and  fifty 
shorter.  Now  if  a  little  extra  height  were  of  udvantage, 
many  or  most  of  the  fifty  would  experience  it ;  though  some 
might  not.     In  the  same  way  if  we  grouped  one  hundred 

Q)  An  eminent  i^igeon  fancier,  Sir  J.  Sebright,  told  ilr.  Darwin  that  he 
could  produce  any  given  feather  in  three  years. 


270 


A    MANUAL    OF   DENTAL    ANATOMY. 


animals  whose  teeth  varied  a  little  in  respect  of  strength  into 
the  fifty  weaker  and  the  fifty  stronger,  it  is  easy  to  see  that 
the  stronger  fifty  would  get  the  better  of  the  others  in  the 
struggle  for  existence  on  the  whole,  and  would  be  more 
certain  to  propagate  their  kind,  and  would  repeat  in  most  of 
their  pi'ogeny  those  peculiarities  which  had  helped  them- 
selves to  live. 

Thus  the  doctrine  of  natural  selection  or  survival  of  the 
fittest,  is  as  fully  applicable  to  the  teeth  of  an  animal  as  to 

Fig.  113  (i). 


.^:: 


any  part  of  its  organisation,  and  the  operation  of  this  natural 
law  will  1)0  constantly  tending  to  produce  advantageous  or 
"  adaptive "  differences.  On  the  other  hand,  the  strong 
power  of  inheritance  is  tending  to  preserve  even  that  which 
in  the  altering  conditions  of  life  has  become  of  very  little 
use,  and  thus  rudimentary  teeth  we  may  understand  to  be 
teeth  which  are  in  process  of  disappearance,  having  ceased 
to  be  tiseful  to  their  possessors,  but  which  are  still  for  a 
time  lingering  upon  the  scene.     Some   teeth   have   disap- 


(^)  Skull  of  a  placental  roflent  (Capybara),  showing  general  character  of 
a  rodent's  dentition. 


THE    TEETH    OF   MAMMALS. 


271 


peared  utterly ;  thus  the  upper  incisors  of  Ruminants  are 
gone,  and  no  rudiments  exist  at  any  stage  (')  (see  page  334); 
others  still  remain  in  a  stunted  and  dwindled  form,  and  do 
not  persist  throughout  the  life-time  of  the  animal,  as  for 
instance  the  first  premolars  of  a  horse,  or  two  out  of  the 
four  premolars  of  most  bears. 

Before  leaving  this  section  of  our  subject,  an  instructive 
illustration  of  the  operation  of  these  agencies  may  be  given. 
Fig.  114  (2). 


It  is  very  easy  for  us  to  see  how  a  "  rodeut "  tjj)e  of 
dentition  is  beneficial  to  its  possessor  by  rendering  acces- 
sible articles  of  food  wholly  unavailable  for  creatures  which 

{})  Statements  to  the  contrary  have  been  made,  and  copied  from  book  to 
book  wthout  verification. 

('^)  A.  Milk  teeth  of  the  Lemurine  Cheiromj's,  with  the  permanent  in- 
cisors just  coming  into  place.  It  diflfers  from  any  Eodent  by  having  many 
milk  teeth,  i.  Permanent  incisor.  i  2.  Posterior  deciduous  incisor. 
c.  Deciduous  canine,  d,  d  2.  Deciduous  molars.  I.  Lower  permanent 
incisor.  I  2.  .Lower  deciduous  cauine.  d  a,  d  b.  Lower  deciduous  molar. 
B.  Reduced  outline  figure  of  its  permanent  dentition,  in  which  it  closely 
mimicks  the  true  rodents. 


272  A    MANUAL    OF   DENTAL    ANATOMY. 


have  no  means  of  gnawing  through  a  shell  or  other  hard 
body.  Now  it  happens  that  in  three  regions  of  the  world, 
pretty  completely  cut  off  from  one  another,  three  animals, 
in  parentage  widely  dissimilar,  have  arrived  at  dentitions  of 
"  rodent "  type. 

Thus  in  Australia,  a  region  practically  wholly  monopo- 
lised by  Marsupials ;  a  marsupial,  the  Wombat,  has  a  den- 
tition very  much  like  an  ordinary  placental  Rodent.  In  the 
island  of  Madagascar,  one  of  the  very  few  parts  of  the  globe 
without  indigenous  rodents,  except  a  fcAv  Muridse,  a  Lemurine 
animal,  the  CheiromyvS,  has  a  dentition  modified  in  a  similar 
direction,  though  probably  employed  to  get  at  a  different 
food ;  and  elsewhere,  scattered  all  over  the  world,  we  have 
the  ordinary  Rodents. 

In  fact,  three  creatures,  as  widely  different  from  each  other 
in  parentage  as  they  well  could  be,  have  been  modified  by 
natural  selection  until  they  have  dentitions,  not  identical, 
but  for  practical  purposes  not  unlike. 

It  is  impossible  to  conceive  that  these  three  creatures 
have  had  anything  in  the  way  of  common  origin  :  their 
ancestry  must  have  been  widely  different,  the  regions  in 
which  they  live  have  been  isolated  from  one  another  for 
countless  years,  and  yet  they  have  each  got  to  a  "rodent" 
type  of  dentition.  Of  extinct  Lemui's  little  is  known,  and 
of  the  ancestry  of  Cheiromys  nothing ;  but  in  the  compact 
oi-oup  of  Marsupials,  still  living  in  Australia,  we  are  able  to 
dimly  see  some  of  the  progressive  steps  which  seem  to  tend 
towards  a  rodent  form  of  dentition.  In  Australia,  roughly 
speaking,  there  were  nothing  but  Marsupials ;  in  Madagascar 
more  Lemurs  than  anything  else ;  and  in  each  case  out  of 
the  material  at  hand,  natural  selection  manufactured  its 
"  rodent "  dentition. 

At  the  same  time  the  force  of  inheritance  is  seen  in  each 
of  them  retaining  characteristics  of  the  groups  whence  they 
have    been    derived,    so    that    underlying    the   priind  facie 


THE    TEETH    OF   MAM3IALS.  273 

resemblance  in  the  teeth,  there  iire  points  in  their  several 
dentitions  whereby  the  wombat  shows  its  marsupial  affini- 
ties, and  the  Aye-aye  its  quadinimanous  affinities. 

In  addition  to  those  modifications  which  are  of  direct  use 
to  the  individual  in  the  way  of  assisting  in  the  procuring  of 
food,  &c.,  any  character  which  would  enable  one  male  to  get 
an  advantage  over  other  males,  and  so  render  him  more 
certain  to  propagate  his  kind,  will  be  sure  to  be  transmitted 
and  intensified. 

Thus  we  can  understand  how  the  males  of  some  species 
have  become  ornamented ;  how  the  males  of  many  birds 
have  come  to  sing  :  and,  what  is  of  more  immediate  concern 
to  us,  how  the  males  of  some  animals  have  become  possessed 
of  weapons  which  the  females  have  not.  The  possession  of 
weapons  by  the  male  is  strikingly  exemjilified  in  the  teeth 
of  animals.  The  males  of  many  frugivorous  monkeys  have 
canine  teeth  much  larger  than  those  of  the  females ;  they 
are  cut  late  coincidently  with  the  attainment  of  sexual 
maturity,  and  are  useful  to  their  possessors  as  weapons  in 
their  combats  with  other  males.  The  male  narwal  has  its 
single  elongated  tusk;  the  male  dugong  has  tusk-like 
incisors;  in  the  respective  females  these  same  teeth  ai'e 
insignificant. 

But  the  most  striking  instance  of  the  teeth  being  modi- 
fied, so  as  to  serve  as  weapons  for  sexual  combat,  is  afforded 
by  some  members  of  the  group  of  ruminants,  amongst  whom, 
as  Cuvier  long  ago  pointed  out,  those  which  are  armed  with 
horns  have  no  canine  teeth,  and  vice  versd — a  generalisation 
which,  although  subject  to  slight  exceptions,  remains  upon 
the  whole  ti'ue. 

The  male  musk-deer  (Moschus  moschiferus)  has  canine 
teeth  of  enormous  lengih,  while  it  is  quite  without  horns 
(see  fig.  115);  the  female  has  no  canine  teeth.  The  male 
muntjak,  which  has  very  short  horns,  has  canine  teeth,  but 
of  much  smaller  size  than  those  of  the  musk-deer.     Other 


2/4 


A    MANUAL    OF   DEXTAL    ANATOMY. 


examples  of  hornless  deer  furnished  with  canine  teeth  are  to 
he  found  in  Swinhoe's  water-deer  (Hydropotes  inermis)  and  in 
the  Elaphodus  cephalophus  (which  has  very  small  antlers), 
a  Chinese  deer  more  recently  discovered,  and  in  the  Tragu- 
Udce.  It  is  obvious  that  males  furnished  with  weapons 
more  jiowerful  than  their  fellows,   will  be   more  likely  to 


LSalS, 


prove  victorious  in  their  battles,  to  drive  away  the  other 
males,  to  monopolise  the  herd  of  females,  and  so  to  transmit 
their  own  peculiarities  to  offspring,  which  will  again  be 
favoured  in  the  same  wa3\  Thus  it  is  very  easy  to  see  how, 
amongst  gregarious  animals,  the  development  of  teeth 
serving  as  sexual  weapons  is  likely  to  be  favoured,  genera- 
tion after  generation,  nntil  canines  as  highly  specialised  as 
those  of  the  musk-deer,  or  the  wild  boar,  are  attained  to. 

It  will  suffice  to  indicate  to  the  reader  that  he  must  be 
prepared  to  find  that  the  teeth  are  profoundly  susceptible 
of  modification,  but  that,  amid  all  their  varied  forms,  the 
evidences  of  descent  from  ancestors  whose  teeth  departed 
less  from  the  typical  mammalian  dentition  are  clearly  trace- 
able by  the  existence  of  rudimentary  teeth  and  other  such 

(^)  Cranium  of  Moschus,  showing  tlie  long  canine  tcotli. 


THE    TEETH    OF   MAMMALS.  275 


characters.  And,  althougli  it  is  by  no  means  probable  that 
we  have  recognised  more  than  a  part  of  the  agencies  which 
are  at  work,  natural  selection  and  sexual  selection  appear 
to  be  competent  to  produce  most  of  the  phenomena  of 
modification  observed.  There  remains  one  other  influence, 
much  more  obscure  in  its  nature,  to  be  touched  upon, 
namely,  "  correlation  of  growth "  or  "  concomitant  varia- 
tion." "When  we  fi)id  that  when  horns  are  developed,  canine 
teeth  are  absent ;  or  that,  after  a  boar  has  been  castrated, 
his  tusks  cease  to  grow,  although  we  may  be  quite  imable 
to  conceive  the  precise  manner  in  which  the  one  thing  in- 
fluences the  other,  we  can  see  that  there  is  a  consistency  in 
the  development  of  the  sexual  weapon  ceasing  coincidently 
with  the  destruction  of  the  sexual  apparatvis,  or  in  the  fact  that 
two  kinds  of  weapon  are  not  developed  in  the  same  animal. 

But  there  are  some  correlations  of  growth  of  a  still  more 
recondite  nature,  in  which  the  connection  is  less  obvious. 
Of  this  nature  is  the  relation  which  exists  between  pecu- 
liarities of  the  skin  and  of  the  teeth  :  the  Edentata,  abnormal 
in  their  skins,  are  different  from  most  other  Mammalia  in 
their  teeth ;  fa^tal  Avhales,  yet  more  aberrant  in  the  nature 
of  their  skins,  have  only  rudimentary  teeth,  in  tlic  place  of 
which,  after  birth,  plates  of  whalebone  are  found. 

Mr.  Darwin  ("Animals  and  Plants  under  Domestication,") 
has  collected  a  number  of  curious  instances  of  relations  exist- 
ing between  hair  and  teeth.  In  general  terms  it  may  be  said 
that  any  great  abnormality  in  the  hair  goes  hand  in  hand 
with  an  abnormalit}-  of  the  teeth.  Thus,  there  is  a  breed  of 
dogs  found  in  Turkey  which  are  almost  hairless,  and  which 
have  very  few  teeth,  their  dentition  being  red  need  to  a  single 
molar  on  each  side,  together  with  a  few  imperfect  incisors  ; 
and  in  the  human  subject  inherited  baldness  has  been  found 
to  be  associated  with  inherited  deficiency  of  the  teeth. 

But  Avp  must  not  go  further  than  to  say,  that  great 
abnormality   of  hair  goes  hand   in  hand   with  abnormality 

T   2 


276  A    MANUAL    OF   DENTAL    ANATOMY. 


of  teeth,  for  examples  have  just  been  i^iveu  of  absence  of 
hair  and  absence  of  teeth ;  and,  on  the  other  hand,  redund- 
ance of  hair  has  in  several  cases  been  accompanied  by 
absence  of  teeth. 

Thus,  in  the  case  of  the  now  famous  hairy  family  of 
Bui'mah,  the  j)eculiarity  of  silky  hair  being  developed  over 
the  face  Avas  transmitted  to  a  third  generation,  and  in  each 
case  the  teeth  were  very  deficient  in  number,  A  year  or 
two  ago  a  hairy  man  and  his  son,  said  to  have  come  fi'om 
the  interior  of  Russia,  were  exhibited  in  London,  and  they 
were  also  almost  toothless.  Q) 

A  good  many  years  ago  a  hairy  woman  (Julia  Pastrana)' 
was  exhibited  in  London,  of  whom  it  has  commonly  been 
reported  that  she  had  an  extensive  nvimber  of  teeth.  Certain 
it  is  that  her  mouth  was  very  prominent,  and  that  she  was 
described  as  "  dog-faced"  and  "pig-faced,"  but  models  have 
been  presented  to  the  Odontological  Society  by  jNIr.  Hepbuna, 
which  are  indisputably  known  to  be  models  of  her  mouth, 
and  these  do  not  show  any  excessive  number  of  teeth.  The 
teeth,  at  least  such  of  them  as  can  be  seen,  are  enormously 
large,  but  the  mouth  is  aflected  with  general  hypertrophy 
of  the  gums  and  alveolar  j^rocesses  to  such  a  degree,  that 
only  a  few  of  the  teeth  can  be  made  out. 

But  this  does  not  make  her  case  the  less  interesting  to 
the  odontologist,  for  in  the  huge  teeth,  the  enormous  palillte 
of  the  glim,  and  the  redundant  hau's  on  the  face,  we  have 
evidence  of  a  disposition  to  hypertrophies  of  the  integument 
affecting  in  different  places  the  different  tegumentary  appen- 
dages which  happen  to  be  there.     And  that  the  teeth  are 

(1)  The  man's  moutli  exemplified  the  dependence  of  the  growth  of  the 
jaw  upon  the  presence  of  teeth.  Ordinarily  the  increase  in  size  between 
childhood  and  adult  age  takes  place  by  a  backwai-d  elongation,  -nhich 
allows  for  the  successive  development  and  eniption  of  the  molars  behind 
the  sjjace  occui^ied  by  the  temporary  teeth.  But  this  man  never  had  any 
true  molars,  and  no  such  backward  elongation  of  the  jaw  had  ever  taken 
place,  so  that,  though  he  was  a  full-sized  man,  his  jaw  was  no  larger  than 
a  child's. 


THE    TEETH    OF   MAMMALS.  277 


dermal  appendages  has  been  shown  at  a  previous  page  (see 
page  2). 

He  would  indeed  be  a  rash  man  -who  ventured  to  assert 
that  we  had  recognised  all  the  agencies  which  are  at  work 
in  the  modelling  of  animal  and  vegetable  forms  ;  l)ut  it  is 
safe  to  say  that,  at  the  present  time,  we  are  acquainted  with 
"  natural  selection,"  or  "  sui-vival  of  the  fittest,"  an  agency 
by  which  variations  beneficial  to  their  possessors  will  be 
l^reserved  and  intensified  in  successive  generations ;  of 
"  sexual  selection,"  which  operates  principally^  by  enabling 
those  possessed  of  certain  characters  to  pi-opagate  their  race, 
while  others  less  favoured  do  not  get  the  opportunity  of  so 
doing ;  of  "  concomitant  variation  "  between  different  parts 
of  the  body,  an  agency  much  more  recondite  in  its  opera- 
tions, but  by  which  agencies  affecting  one  pai*t  may  second- 
arily bring  about  alterations  in  some  other  part. 

And  operating  in  the  contrary  direction,  we  have  a  certain 
fixity  of  organisation,  so  that  the  power  of  inheritance  is 
constantly  asserting  itself  by  the  retention  of  parts  which 
have  become  useless,  for  a  time  at  all  events,  and  by  the 
occasional  reappearance  of  characters  which  have  been  lost. 

Allusion  has  been  made  to  these  great  biological  questions 
with  the  view  of  helping  the  student  to  have  patience  to 
master  descriptions  of  minute  points,  of  which  he  does  not 
at  the  moment  see  the  bearing,  by  giving  him  confideuce 
that  there  are  no  characters  so  trivial  but  that  they  may 
throw  very  important  light  upon  the  remote  parentage  and 
the  line  of  descent  of  the  creature  under  examination.  And 
as  a  further  incentive  to  painstaking  and  minute  observa- 
tion, it  may  be  added,  that  things  which  are  rudiraentarj- 
and  therefore  inconspicuous,  are  often  just  the  things  which 
happen  to  teach  us  most ;  for  being  of  no  present  use,  they 
are  not  undergoing  that  rapid  change  in  adaptation  to  the 
creature's  habits  which  may  be  going  on  in  organs  which  are 
actively  employed. 


278  A    MANUAL    OF    DENTAL    ANATOMY. 


THE    ]IOMOLOGIK.S    OF    THE    TEIJTH. 

A  superficial  survey  of  the  teeth  of  tliose  mammals  which 
possess  two  sets  of  teeth  (diphyodonts)  will  indicate  that, 
notwithstanding  the  apparent  anomalies  brought  about  by 
adaptive  modifications,  a  close  correspondence  between  the 
several  teeth  of  different  animals  exists.  That  is  to  say,  we 
can  generally  identify  incisors,  premolars,  and  molars ;  nay, 
more,  when  an  animal  has  less  than  the  full  typical  number 
of  a  particular  class  of  teeth,  we  can  ordinarily  say  with 
certainty  which  of  them  it  is  that  are  absent. 

As  it  is  impossible,  or  at  least  inconvenient,  to  avoid  the 
use  of  the  term  "typical"  dentition,  it  will  be  well  to  explain 
at  the  outset  what  is,  and  what  is  not,  meant  by  it. 

That  the  gTeat  majority  of  biologists  reject  utterly  the 
"  archetype  "  theory,  by  which  all  those  resemblances  which 
really  exist  were  refeired  to  the  influence  of  a  sort  of  gene- 
ralised "  pattern "  animal,  according  to  the  model  of  which 
all  other  animals  were  fashioned,  has  already  been  mentioned : 
this,  then,  is  what  is  not  meant  by  a  "typicol"  dentition. 
What  is  meant,  is  a  form  so  simplified,  so  little  modified  in 
in  any  special  direction,  that  we  can  conceive  it  to  be  near 
to  a  common  parent  form  Avhencc,  by  progressive  modification 
in  successive  generations,  other  forms  have  been  derived. 
We  cannot  point  to  any  mammalian  dentition  at  present 
known  to  us,  and  say  this  may  have  been  the  parent ; 
tliis  is  a  typical  form  of  mammalian  dentition  ;  but  we 
do  know  many  fossil  forms  which  approximate  to  it  far 
moi-e  closely  than  do  any  at  present  in  existence,  and  as 
transitional  forms  of  animals,  and  animals  of  highly  gene- 
ralised characters,  are  every  day  coming  to  light,  we  do  not 
doubt  that  such  forms  once  did  actually  exist,  and  may 
one  of  these  days  be  found.  Absolute  proof  would  be  ob- 
tainable only  if  we  could  refei-  to  its  place  every  mammal 
that  had   ever  existed,  and   show  every  step  in  the   series 


THE    TEETH    OF  MAMMALS. 


of  modifications  by  which  the  ultimate  divergence  of  den- 
tition Avas  efiected.  But  evidence  far  short  of  absokite 
ocuhxr  demonstration  serves  to  satisfy  vis  on  most  points, 
and  there  is  sufficient  evidence  available  to  enable  us  to 
say  with  some  confidence  that  our  "tyiMcal"  or  parent 
mammalian  dentition  was,  so  far  as  the  numbers  of  the 
several  kinds  of  teeth  go, 

.3        1  4        3        ,  , 

a    -  c      -  prni        m  ,^  zz  4-i. 

And  when  there  are  less  than  forty-four  teeth,  as  has  been 
already  mentioned,  we  can  in  most  cases  say  which  they 
are  that  are  absent. 

Thus,  taking  a  certain  bear  and  a  baboon  {each  having 
two  premolars  only  on  each  side),  we  are  able  to  decide,  by 
comparison  with  allied  creatures,  that,  in  the  case  of  the 
bear,  it  is  the  second  and  third  premolars  which  are  wanting, 
the  first  and  fourth  remaining ;  while  in  the  baboon  it  is  the 
first  and  second  which  arc  wanting,  the  third  and  fourth 
being  present.  By  homology  we  mean  such  correspondence 
as  is  above  indicated ;  a  correspondence  which  might  almost 
be  expressed  as  a  relationship  by  descent. 

Homology,  then,  is  almost  equivalent  to  identity  of  oi-igin, 
or,  at  all  events,  to  similarity  of  origin ;  but  it  by  no  means 
necessai-ily  involves  identity  or  even  similarit}*  in  the  pur- 
pose to  which  a  thing  is  ultimately  applied — a  fact  which  will 
be  further  illustrated  in  speaking  of  canine  teeth. 

The  homologies  of  the  teeth  may  be  treated  under  two 
heads  :  the  one,  the  homologies  of  the. teeth  in  their  relation 
to  other  parts  of  the  body,  and  the  other,  their  more  special 
homologies,  or  their  relation  to  one  another. 

The  relation  of  the  teeth  to  the  skin,  which  we  express  by 
calling  them  "  dermal  ai^pendages,"  as  well  as  the  epidermic 
nature  of  the  enamel,  and  the  dermic  nature  of  the  dentine, 
have  been  sufiiciently   discussed   at  former  pages,  so    that- 


A    MANUAL    OF   DENTAL    ANATOMY. 


\vc  may  at  once  pass  to  the  homologies  of  the  teeth  with 
one  aiiotlier. 

Teeth  are  divided  into  incisors,  canines,  ])remokrs,  and 
molars,  but  these  classes  do  not  all  admit  of  quite  satis- 
factory definition.  Incisors  are  defined  as  teeth  implanted 
in  the  intermaxillaiy  bone,  a  definition  which  has  the  merit 
of  being  precise ;  and  on  the  whole  there  is  a  certain  resem- 
blance running  through  incisor  teeth  in  most  animals,  but 
the  definition  of  lower  incisors  as  being  the  corresponding- 
teeth  in  the  lower  jaw  is  a  good  deal  less  satisfactory,  because 
they  are  not  situate  upon  any  distinct  bone.  And  it  has 
even  been  denied  that  there  can  be  a  true  homology  between 
a  maxillary  and  a  mandibular  tooth. 

Molars  arc  teeth  at  the  back  of  the  mouth,  which  come 
up  behind  the  milk  teeth  (when  there  are  any),  and  which 
are  generally  subservient  to  grinding  the  food. 

Premolars  are  teeth  in  front  of  the  molars,  usually  differ- 
ing from  them  by  being  more  simple  in  form  and  being- 
smaller,  and  in  most  animals  by  having  displaced  deci- 
duous predecessors.  But  they  are  not  always  simpler  in 
form,  nor  smaller  (e.ff.,  the  horse,  fig.  138),  nor  do  they 
always  displace  deciduous  predecessors  (e.o.,  they  do  not  all 
do  so  in  the  Marsupials),  so  that  this  definition  is  not  abso- 
lutely j)recise.  Still,  as  a  matter  of  practice,  it  is  usually 
easy  to  distinguish  the  premolars,  and  the  division  into 
premolars  and  molars  is  useful. 

Any  objection  that  can  be  raised  to  the  name  of  premolar 
on  the  score  of  a  short  logical  definition  being  impossible, 
applies  with  tenfold .  force  to  the  canines.  (Cf.  Messrs. 
Mosely  and  Lankester,  .Journ.  Anat.  and  Physiology,  1869.) 

The  neai'est  approach  to  a  good  definition  is  that  which 
describes  the  canine  as  the  next  tooth  behind  the  intermax- 
illary suture,  provided  it  be  not  far  behind  it;  and  the 
lower  canine  as  the  tooth  which  closes  in  front  of  the  upper 
canine. 


THE    TEETH    OF  MAMMALS.  281 


A  great  deal  of  coufusiou  has  arisen  out  of  the  twofold 
sense  iu  -which  the  word  "  canine  "  is  used  :  if  it  were  always 
applied  to  designate  the  first  tooth  in  the  maxilla  of  the 
typical  mammalian  dentition  quite  irrespective  of  its  size, 
etc.,  and  of  the  lower  tooth  closing  in  front  of  it,  no  objec- 
tion to  its  employment  could  be  made,  inasmuch  as  it  wovild 
designate  truly  homological  organs. 

But  it  so  happens  that  the  tooth  in  question  is,  in  a  very 
large  number  of  familiar  animals,  developed  to  a  large  size 
and  sharply  pointed  for  use  as  a  weapon,  and  so  with  the 
word  canine  there  comes  to  be  associated  a  teleological 
idea;  and  hence  we  are  dissatisfied  with  calling  the  first 
maxillary  tooth  "  canine,"  when  it  is  some  other  tooth 
which  is  doing  its  work. 

On  the  other  hand,  if  we  are  to  leave  out  of  court  all 
considerations  as  to  size,  purpose  to  which  it  is  to  be 
applied,  and  so  forth,  there  is  nothing  left  to  make  it 
deserving  of  a  name  distinguishing  it  from  the  four  teeth 
behind  it.  So  we  must  be  content  with  some  such  state- 
ment as  the  following. 

A  veiy  large  number  of  animals,  notably  the  Carnivora, 
have  one  tooth,  situated  a  little  way  from  the  front  of  the 
mouth,  developed  to  an  unusual  length  and  sharply  pointed, 
for  use  as  a  weapon.  Tlie  tooth  which  has  undergone  this 
adaptive  modification  is  usually  the  first  which  lies  in  the 
maxillary  bone ;  in  fact,  the  foremost  of  the  premolar 
series  ;  but  it  occasionally  happens  that  it  is  some  other 
tooth  which  has  undergone  this  modification.  When  we 
use  the  term  canine  we  should  generally  mean  a  tooth  so 
modified,  and  generally,  but  not  always,  should  be  alluding 
to  the  same  tooth,  i.e.,  to  the  tooth  which  in  the  typical 
mammalian  dentition  comes  next  behind  the  outermost  or 
incisor — the  first  of  the  premolars,  if  we  allow  five  premolars 
instead  of  four. 

It  would  practically  be  very  inconvenient  to  abolish  the 


282  A    MAXUAL    OF   DENTAL    ANATOMY. 

term  canine  ;  but  it  .should  be  borne  in  inind  tliat  its  signi- 
ficance is  mei'ely  equivalent  to  "  caninifonu  premolar,"  and 
that  in  describing  the  dog's  dentition  (fig.  lG3)we  should  be 
less  liable  to  be  misinterpreted,  were  we  to  say  that  it  has 
five  premolars,  of  which  the  first  is  caniniform.  To  those 
who  accept  the  doctrine  of  evolution  it  is  not  needful  to  say 
more,  as  it  is  hardly  possible  to  resist  the  conclusion  that 
the  teeth  of  the  parent  forms  were,  like  those  of  the  present 
monophyodonts,  not  much  differentiated  from  one  another. 
Then,  as  animals  diverged  and  became  modified  in  accordance 
with  their  requirements,  their  teeth  would  become  so  far 
difiin-entiated  that  they  would  admit  of  being  classified. 
Thus  the  Carnivora  would  have  attained  to  a  stage  of 
diff"erentiation  in  which  the  canine  is  functionally  certainly 
deserving  of  a  distinction,  whereas  along  other  lines  of 
descent,  differentiation  having  not  proceeded  so  far,  or  having 
proceeded  in  a  somewhat  different  direction,  it  would  not 
merit  a  distinctive  appellation. 

But  as  it  so  happens,  that  all  the  works  on  odontography 
have  started  upon  the  basis  that  there  was  a  ''  type  "  denti- 
tion, and  as  a  canine  figured  in  that  dentition,  it  will  be 
necessary  to  point  out  a  few  instances  of  the  propositions  to 
which  those  anatomists  are  committed  who  call  some  tooth 
a  "  canine  "  in  every  case  where  a  tooth  is  situated  in  the 
maxillary  bone,  close  behind  the  suture  which  connects  it 
with  the  intermaxillary  bone,  whether  that  or  any  other 
tooth  be  large  and  pointed,  "  caniniform  "  or  not. 

In  typical  Ruminants,  the  upper  jaw  lacks  both  incisors 
aiid  canines  (with  certain  exceptions,  for  which  see  p.  335),  but 
in  front  of  the  lower  jaw  there  are  grouped  together  eight 
teeth,  closely  fitted  together,  and  of  almost  exactly  similar 
size  and  shape.  The  outermost  pair  of  these  teeth  are 
called  canines,  because  (i.)  in  some  allied  species  the  tooth 
in  this  situation  is  more  pointed;  (ii.)  because  this  tooth 
shuts  in  advance  of  the  upper  canine  when  the  mouth  is 


THE    TEETH   OF  MAMMALS. 


closed  (iu  those  allied  creatures  -wliicli  have  au  undoubted 
upper  cauine) ;  (iii.)  because  it  is  cut  later  than  the  others 
(Owen). 

These  three  reasons  arc  weak,  because  (i.)  form  is  a 
veiy  unsafe  guide  to  homology,  and  as  to  the  lateness  of  its 
development  (iii.),  it  succeeds  to  the  third  incisor,  by  Pro- 
fessor Owen's  own  showing,  after  about  the  same  lapse  of 
time  which  separated  the  eruption  of  the  second  and  third 


incisors.  Moreover,  Oreodon,  an  extinct  Ruminant  with 
caninifonn  teeth,  has  the  eight  incisors  in  the  lower  jaw  in 
addition  to  a  caniniform  tooth,  ivhich  is  the  fifth  tooth  coimtiwj 
from  the  front.  With  reference  to  the  relative  positions  of 
the  upper  and  lower  teeth,  determining  which  is  and  which 
is  not  "the  canine,"  (ii.)  no  one,  looking  at  the  dentition 
of  Oreodon,  would  be  inclined  to  hesitate  which  teeth  he 
should  call  "■  canines ; "  yet  the  lower  caniniform  tooth 
shuts  behind  the  upper,  and  therefore,  according  to  this 
test,  it  is  not  a  true  canine. 

In  the  Lemurs  there  are  similarly  eight  procumbent  teeth 


(')  Oreodon  Culbertsonii  (after  Leidy).  It  will  Le  observed  tliat  iu  tlic 
Tipper  jaw  the  four  premolars  of  the  typical  mammalian  dentition  are 
behind  the  "cauine,"  but  that  in  the  lower  jaw  the  tooth  which  would 
fulfil  the  functions  of  a  canine  is  the  first  of  these  four,  and  therefore  is 
not  the  corresponding  tooth  to  the  "  canine"'  in  the  upper  jaw. 


284  A    MAXUAL    OF   DENTAL    ANATOMY. 


occupying  the  front  of  the  lower  jaw,  of  which  the  outer- 
most pair  are  called  canines,  although  not  in  the  smallest 
degree  meriting  that  name  for  any  other  reason  than  that 
they  close  in  front  of  the  caniniform  tooth  of  tlie  upper  jaw, 
for  they  are  just  like  the  other  incisors. 

But  it  is  in  the  Imectlrora  that  the  greatest  difficulties 
occur. 

To  the  mole  no  less  than  four  dental  formulae  have  been 
assigned,  all  turning  upon  the  identification  of  the  canine. 
The  difficulty  is  this  :  The  upper  tooth,  which  looks  like  a 
canine,  has  two  roots,  and  is  implanted  (and  its  deciduous 
predecessors  also  lie)  (Spencer  Bate)  within  the  limits  of 
the  premaxillary  bone.  And  besides  this,  the  lower  tooth, 
which  answers  the  purpose  of,  and  looks  like,  a  canine, 
closes  behind  instead  of  in  fi'ont  of  the  great  upper  tooth. 

Fig.  117  C). 


ErictiJm  has  a  sharp,  long-,  two-fanged  tooth,  in  pattern  of  crown, 
an  enlarged  premolar,  in  position  of  upper  canine,  and  no  canini- 
form  tooth  in  lower  jaw. 

Ceiitefcs  has  tyiiical  canines,  like  a  Cai-nivore. 

Ilcmlccntctcit.  the  so-called  canine,  differs  in  no  respect  from  the 
premolars  behind  it. 

Ei-iiiacnix.  So  called  ujiper  canine  two-rooted,  aud  like  the  pre- 
molars which  follow  behind  it. 

(')  Upper  and  lower  teetli  of  the  common  mole.  lu  it,  just  as  in 
Oreodon,  the  teeth  which  fulfil  the  functions  of  canines  are  not  corre- 
sponding teeth  in  the  upper  and  lower  jaws. 


THE    TEETH    OF   MAMMALS. 


Gj/mnura.    Upper  canine-like  tooth  has  two  roots  ;  a  single-rooted 

lower  pointed  tooth  closes  in  front  of  it. 
Maeroncclis  and  Pctrofh-oiinis.     The  third   or  outermost  incisor  is 

two-rooted,  long,  and  sharp,  and  plays  the  part  of  a  canine. 
PotnmogaJc.    A  small  tooth,  in  no  respect  different  from  the  other 

premolars,  is  called  a  "  canine." 

In  some  of  the  groups  no  tooth  has  been  lengthened  and 
pointed,  so  as  to  serve  as  a  canine ;  in  others  it  is  the  Avrong 
tooth,  i.e.,  not  the  same  tooth  as  in  the  Carnivora,  or  as  in 
other  Insectivora.  Conseqixentl}-,  in  the  Insectivora  the 
elevation  of  a  tooth  into  caniniform  length  and  character  is 
a  mere  adaptive  modification,  which  may  affect  an  incisor, 
or  a  premolai",  or  no  tooth  at  all. 

It  appears  to  me  that  the  result  of  all  investigations  into 
the  homologies  of  mammalian  teeth  may  be  summed  up 
somewhat  in  the  following  manner. 

The  evidence  of  a  common  pattern,  which  is  traceable  in 
incisors,  canines,  i^remolars,  and  molars  (see  ]3age  8),  wop.ld 
seem  to  indicate  that  their  special  forms  have  been  all 
derived  from  modifications  of  some  much  more  simple  form, 
and  that  if  we  are  ever  to  find  what  might  be  called  a  parent 
mammalian  dentition,  it  will  be  nearly  "  homodont : "  that 
is  to  say,  the  several  teeth  will  not  differ  much  from  one 
another  in  size  and  shape,  just  as  we  see  to  be  the  case  in 
the  dolphin  (see  fig.  131),  or  the  armadillo. 

It  becomes  open  to  question  whether  the  term  canine  is 
desirable  in  dental  formulae  or  in  homological  determinations  ; 
if  we  put  on  one  side  its  functional  modifications  there  is 
nothing  left  to  distinguish  it  from  other  premolars  :  if  we 
bear  in  mind  its  functional  development  we  encounter  the 
anomalies  noticed  above. 

If  we  were  able  to  place  in  unbroken  series  all  the  den- 
titions through  which,  by  progressive  modification,  the 
original  almost  homodont  dentition  had  passed  into  a 
highly  specialised  dentition,  like  that,  say,  of  the  cat,  it 
•would  be  a  matter  of  impossibility  to  fix  upon  any  point 


286  A    MANUAL    OF  DENTAL    ANATOMY. 

where  we  should  bo  justified  in  assertiiiu'  that  here  the 
hoinodont  dentition  has  recently  become  heterodont  :  at 
this  point,  for  the  first  time,  we  have  incisors,  canines, 
molars. 

As  a  matter  of  fact,  a  large  number  of  extinct  Ungiilata 
had  the  full  t3'pical  number  of  mammalian  teeth,  viz.,  forty- 
four,  and  in  some  the  individual  teeth,  incisors,  canines, 
premolars,  and  molars,  passed  into  one  another  by  insensible 
gradations,  and  contiguous  teeth  were  but  little  differen- 
tiated from  one  another.  Professor  Flower  has  described 
and  figured  such  an  extinct  Ungulate  under  the  name  of 
Homalodontotherium  (Philos.  Trans.,  1874).  It  is  exceedingly 
interesting  to  find  that  back  in  geological  time  the  dentitions 
were  more  generalised,  both  carnivorous  and  herbivorous 
mammals  of  the  Eocene  period  usually  j^ossessing  the  full 
typical  number  of  teeth,  and  displaying  less  of  special 
modification ;  but  the  few  forms  of  life  which  have  been 
handed  down  in  a  fossil  state  do  not  as  yet  offer  us  by  any 
means  an  unbroken  chain  of  forms  differing  from  one  another 
by  progressive  modification,  except  in  a  few  cases  :  thus  the 
ancesti'y  of  the  horse  is  now  comparatively  completely  known 
to  us.  Bearing  in  mind  that  the  several  kinds  of  teeth  have 
probably  a  common  origin,  the  homological  differentiation 
in  the  incisors,  premolars,  and  molars  may  be  advantageously 
admitted,  and  made  use  of  as  a  basis  for  comparing  and 
classifying  the  teeth  of  different  animals.  It  is  usually  said 
that  when  incisors  are  missing  from  the  full  typical  number, 
they  are  lost  from  the  outer  end  of  the  series :  that 
is  to  saj',  if  there  is  but  one  incisor  it  is  Ij^  ;  if  two,  I^ 
and  L. 

There  are  many  exceptions  to  this  :  e.g.,  the  first  incisor 
is  the  first  to  disappear  in  the  otter,  walrus,  and  some  few 
others. 

When  premolars  are  missing,  it  is  said  that  they  are  lost 
from  the  front  of  the  scries.     This  is  generally  true,  but  the 


THE    TEETH    OF   MAMMALS!. 


following  exceptions  may  be  given.  In  bears  the  second 
premolar  is  often  absent,  the  first  being  very  constant ;  the 
same  thing  is  true  of  many  bats ;  in  Dasyurus  the  third,  or 
hindmost  (it  being  a  ]\Iarsupial)  is  absent,  the  first  two  lacing 
present.  Q) 

A  difiiculty  at  times  occm-s  in  deciding  whether  a  tooth 
is  to  be  regarded  as  a  premolar,  or  as  a  milk  tooth,  as  there 
are  many  so-called  permanent  teetli  which  are  lost  early  in 
the  lifetime  of  the  animal. 

Professor  Flower  gives  an  instance  of  this  in  the  hippo- 
potamus :  the  first  premolar  appears  with  the  milk  teeth  ; 
it  probably  has  no  predecessor,  and  is  shed  in  middle  life. 
But  in  allied  forms  the  corresponding  tooth  remains  in  place 
throughout  the  creature's  life. 

The  wart-hog  is  a  conspicuous  example  of  the  early  loss 

of  teeth  which  clearly  belong  to  the  permanent  series  (see 

page  328),  all  the  teeth  (premolar  and  molar)  in  front  of  the 

last  great  molar  being  cast  off,  and  the  dentition  ultimately 

reduced  to — 

.21         1 

1  _    c  ^  m  _ 

:'.        1  1 

That  general  correspondence,  whicli  is  found  to  exist 
between  the  dentitions  of  various  animals,  extends  also  to 
the  patterns  of  individual  teeth,  so  that  we  are  able  to  trace 
out  the  various  stages  by  which  complexity  of  pattern  has 
been  arrived  at. 

In  what  might  be  termed  a  typical  tooth  we  should  have 
a  single  central  pulp  cavity  surrounded  by  a  body  of  hard 
dentine ;  over  the  crown  this  is  coated  by  enamel,  whilst  the 
whole,  crown  and  root,  would  be  invested  b}^  a  layer  of 
cement. 

The  layer  of  coronal  cement   may  Vie  so   thin   as  to  be 

(1)  This  is  ascertained  by  tlje  examination  of  allied  forms,  in  which  the 
hird  premolar  is  found  to  lie  so  small  as  to  l>e  rudimentary. 


288  A    MANUAL    OF  DENTAL    ANATOMY. 


merely  rudimentary,  as  in  Man  or  the  C'arnivora  ;  or  the 
investment  -with  enamel  may  be  only  partial,  as  upon  the 
front  of  a  Rodent  incisor;  or  a  tooth  may  be  composed 
solely  of  a  mass  of  hard  vinvascular  dentine,  as  in  the  teeth 
of  the  Wrasses. 

And  just  as  endless  varieties  of  teeth  may  be  produced 
by  the  suppression,  or  partial  svippression,  of  certain  of  the 
tissues,  so  differences  may  be  brought  about  by  the  occvir- 
rence  of  other  than  the  three  usual  tissiies.  Thus  the 
remains  of  the  central  pulp  cavity  often  becomes  occupied 
by  calcified  pulp,  forming  "  osteodentine ; "  this,  which 
occurs  in  man  as  an  almost  pathological  condition,  is  per- 
fectly normal  in  many  animals ;  in  the  sperm  whale,  for 
instance,  or  in  the  constantly  growing  teeth  of  the  sloth,  the 
central  axes  of  which  are  occupied  by  dentine  permeated  by 
medullary  canals. 

It  is  not  so  much  the  complexities  induced  by  variation 
in  minute  structure  that  concerns  ns  here,  as  those  brought 
about  by  the  arrangement  of  the  different  tissues. 

If  we  take  a  simple  conical  tooth  with  one  cusp,  such  as 
a  canine,  and  grind  or  wear  down  its  apex  till  the  terminal 
portion  of  enamel  is  removed,  its  blunted  end  will  present  a 
more  or  less  circular  area  of  dentine,  surrounded  by  a  rim 
of  enamel.  If  Ave  imagine  a  tooth  with  four  long  similar 
cusps,  we  shall  at  a  certain  stage  of  wear  have  four  such 
areas,  while  eventually,  as  the  tooth  gets  worn  down  below 
the  level  of  the  basis  of  the  cusps,  there  will  come  to  be  a 
single  larger  area  of  dentine  surrounded  by  enamel.  Thus 
in  those  teeth  the  grinding  surfaces  of  which  are  rendered 
complex  in  pattern  by  the  presence  of  several  cusps,  the 
pattern  changes  from  time  to  time  as  the  tooth  wears  down ; 
while  the  addition  of  thick  cementum  filling  up  the  inter- 
spaces of  the  cusps,  adds  a  further  element  of  complexity,  as 
is  seen  in  the  teeth  of  most  herbivorous  creatures.  The 
change  of  pattern  induced  by  the  wearing  down  of  the  surface 


THE    TEETH    OF   MAMMALS. 


to   a   lower   level   is    well    and    sim])ly    illustrated  by   the 
"  mark  "  of  the  incisor  teeth  of  a  horse. 

Ill  an  uncut,  and  therefore  perfectly  unworn  tooth,  such 
as  is  represented  in  the  figure,  the  condition  of  the  apex 
may  be  compared  to  the  fii^ger  of  a  glove,  the  tip  of  which 

Fig.  118  Q). 


has  been  pushed  in  or  invagiiiated.  The  depression  so 
formed  is,  like  the  rest  of  the  sin-face,  coated  with  enamel, 
and  with  a  thin  layer  of  cementum. 

When  the  tooth  is  worn  down  to  a  considerable  extent, 
we  have  a  field  of  dentine,  in  the  centre  of  which  is  an 
oval  ring  of  enamel;  within  this  a  space  filled  with  the 
debris  of  food,  tfcc.  This  constitutes  the  mark  (see  next 
jjage),  and  as  the  tooth  becomes  further  worn  down,  below 
the  level  of  the  bottom  of  the  pit,  the  mark  disappears, 
and  a  plain  area  of  dentine  results. 

Not  only  may  inflections  of  the  surface  and  of  the  enamel 
take  place  from  the  grinding  surface,  but  they  also  abun- 
dantly occur  upon  the  sides  of  the  tooth.  The  inflection 
of  the  surface,  which  in  the  incisors  of  the  horse  is  of  the 
simplest  possible  form,  may  be  crucifoi-m,  or  variously 
waved  and  broken  up,  thus  leading  to  all  sorts  of  com- 
plications of  surface.  As  the  tooth  becomes  worn,  the 
longitudinal  inflections,  running  in  from  the  sides,  may  also 
be  oblique,  or  variously  waved,  or  they  may  extend  through 
the  entire  width  of  tlie  tooth,  thus  cutting  it  into 


Q)  Apex  of  crown  of  an  upper  incisor  of  a  Horse,  not  yet  completely 
formed. 


A    MANUAL    OF    DENTAL    ANATOMY. 


of  plates  of  dentine  and  enamel,   fused  into  one  tooth  by 
the  cementum  (sec  fig.  12-")). 

Interestinf?  as  liave  been   the    discoveries   made  of  late 


Fio.  119  (' 


Fig.  120  (-). 


years  n\  Mammalian  paleontology,  it  is  not  as  yet  by  any 
means  possible  to  determine  from  what  common  pattern  or 
patterns  all  complex  mammalian  teeth  may  be  considered 
to  have  been  derived ;  though  the  pattern  of  some,  for 
example,  of  the  molars  of  the  horse,  may  be  traced  back 
in  increasing  simplicity  through  a  number  of  parent  forms. 
Enough  has,  however,  been  done  to  indicate  that  by  careful 
study  many  complexities  of  pattern  may  be  referred  to  a 
few  particular  types,  and  thus  may  be  simplified  by  a  com- 
parison with  other  allied  forms,  in  which  essential  charac- 
teristics are  not  masked  by  minor  complications.     Starting 


(')  Horse  incisor,  in  longitudinal  section. 

(-)  Horse's  incisors,  showing  the  mark  at  various  ages. 


THE    TEETH    OF   MAMMALS.  291 


from  the  human  tooth,  as  being  familiar  to  us  all,  a  quadrate 
crown,  with  four  cusps  at  its  corners,  is  common  to  many 
animals ;  the  oblique  ridge,  present  in  some  apes,  not  in 
others,  is  met  with  also  in  some  Insectivora,  e.g.  the  Hedge- 
hog. Ax'ound  the  neck  of  many  teeth  runs  a  pronounced 
ridge,  the  "  cingulum,"  and  tliis  may  be  produced  up  into 
additional  cusps. 

A  very  instructive  series  of  comparisons  of  the  molar 
teeth  of  Insectivora  has  been  made  by  Mr.  jMivart  (Journal 
of  Anat.  and  Physiol.:  1868);  pointing  out  that  within 
the  limits  of  this  group  a  great  variety  of  patterns  is  met 
with,  the  several  modifications  being  connected  by  transi- 
tional forms. 

It  would  appear  that  upon  the  molar  teeth  (npper)  of 
Insectivora  there  are  four  principal  cusps  (lettered  a,  b,  c,  d, 
in  the  figure)  which  are  more  or  less  connected  by  ridges ; 
such  simple  teeth  are  met  with  in  the  elephant  mice 
(Macroscelides),  and  hedgehog.  The  cingulum  is  well 
developed  in  most  of  the  group,  and  the  further  complexity 
of  the  crowns,  which  often  bristle  with  sharp  points,  is 
brought  about  by  the  elevation  of  the  cingulum  into  long 
sharp  points,  equalling,  or  exceeding  in  length,  the  principal 
cusps  of  the  tooth. 

Thus  in  Urotrichus,  a  Japanese  creature  having  affinities 
with  the  mole,  the  external  cingulum  is  elevated  into  thi-ee 
distinct  pointed  cusps,  united  by  ridges  with  the  two  prin- 
cipal cusps,  an  aiTangemeut  which  gives  a  sort  of  W  pattei'n 
to  the  sm-face,  while  to  the  inner  side  the  cingulum  forms 
another  cusp,  so  that  there  ai-e  in  all  eight  cusps;  the 
common  mole  has  the  third  cusp  developed  from  the  outer 
cingulum,  but  its  two  inner  principal  cusps  are  fused  together 
and  lose  their  distinctives.  The  suppression  and  fusion  of 
cusps  is  carried  to  a  much  greater  extent  in  the  compressed 
teeth  of  the  iridescent  mole  (Chrysochloris),  but  there  are 
intermediate    forms  which    render    it    easy  to   identify   its 

x;  2 


A    MANUAL    OF    DENTAL    ANATOMY. 


reversed  part  with  those  corresponding-  to  tlicni  in  the  mole 
or  in  Urotrichns. 

Speaking  generally,  it  may  be  said  that  new  cnsps  are 
added  to   the    number    already  existing,  by  tlie  cingulum 


Fig.  121 


■>*1f 


becoming  elevated  into  points ;  it  is  not  very  unusual  to 
see  subsidiary  cusps,  obviously  originating  in  this  way,  upon 
human  molars. 

Ridges  may  variously  connect  the  cusps ;  and  the  coales- 
cence of  two  or  more  cusps  to  form  an  exceedingly  elevated 
point  is  illustrated  by  the  Carnassial  tooth  of  carnivora ;  to 
this  transformation  certain  marsupial  teeth  form  the  clue,  as 
they  afford  unquestionable  evidence  of  such  coalescence  by 
a  gradational  series  of  small  modifications  in  this  direction 
occurring  in  allied  creatures. 

A  simple  j^attern  of  tooth  is  formed  by  the  junction  of 
the  two  anterior  and  two  posterior  cusj)s  by  simple  ridges ; 
and  the  cingulum  may  connect  the  outer  ends  of  tliese  two 
ridges ;  sucli  a  tooth  is  seen  in  the  Tapir  and  in  the 
Palseotherium.      By  the   varied    obliquity   of  these    ridges, 

C)  Upper  molar  teeth  of  (A)  Urotriclm.s  ;  (IJ)  Mole  ;  and  (C)  Chryso- 
cliloris.  The  four  principal  cusps  are  lettered  a,  b,  c,  d,  in  each  of  the 
figures.  In  A  the  cingulum  has  been  elevated  so  as  to  form  four  additional 
cusps  on  the  exterior  of  the  tooth,  and  one  additional  cusp  on  the  interior. 
B  and  C  show  the  fusion  of  certain  of  these  cusps,  and  the  consequent 
diminution  in  their  number.      (From  Mivart. ) 


THE    TEETH    OF   21  AM  MA  LS. 


and  by  the  introduction  of  secondary  inflections,  patterns 
apparently  dissimilar  are  arrived  at. 

In  the  molar  tooth  of  the  horse,  arrived  at  hy  a  modifi- 
cation of  the  Palseotherium  type,  we  have  a  surface  con- 
stantly kept  rough  by  the  varying  hardness  of  its  difterent 
constituents. 

In  a  worn  tooth,  we  have  upon  a  general  field  of  dentine 


Fi(?.  122  fix 


two  islands  of  cementum,  Iiouuilcd  by  tortuous  lines  of 
enamel,  and  on  the  inner  side  a  sort  of  promontory  of 
dentine,  bounded  by  enamel.  The  tortuous  lines  of  enamel 
by  virtue  of  their  hardness  will,  at  all  stages  of  wear,  be 
more  prominent  than  the  dentine  or  the  cementum,  and 
will  hence  maintain  the  efficiency  of  teeth  as  grinders. 

The  patterns  of  grinding  sm-face  thus  produced,  are  very 
constant  for  allied  species,  so  that  an  individual  tooth  of  a 
herbivore  may  sometimes  be  correctly  referred  to  its  genus, 
and  always  to  its  family. 

But  as  it  will  be  necessary  to  recur  to  this  subject  from 
time  to  time,  it  will  suffice  for  the  present  to  point  out  that 
such  correspondences  do  exist,  and  that  all  the  complexities 
of  pattern  found,  may,  in  practice,  be  reduced  to  some  few 
types. 

(')  Molaf  tooth  of  Horse,  showing  the  cliaract eristic  pattern  of  its 
grinding  surface. 


294 


A    MANUAL    OF   DENTAL    ANATOMY. 


Tlic  development  of  additional  cusps  from  up-growths  of  the 
cingulum,  and  the  suppression  or  fusion  of  pre-existing  cusps, 
may  be  traced  b}'  a  comparison  of  the  teeth  of  allied  animals, 
and  thus  connecting  links  are  foimd  between  jjatterns  at 
first  sight  very  dissimilai-.  The  order  Proboscidea  affords, 
however,  so  instructive  an  example  of  the  manner  in  which 
an  exceedingly  complex  tooth  lias  l)een  derived  from  a 
simple  one,  that  it  may  l)e  mentioned  in  this  place  us  an 
exam]>le. 

The  tooth  of  the  elephant  is  so  strikingly  unlike  other 
teeth  tliat  it  might  at  first  sight  be  supposed  that  it  is 
more  essentially  different  than  is  really  the  case.     The  clue 


Fig.  123  ('). 


bed  e 


to  its  nature  is  afforded  by  the  teeth  of  an  extinct  Pro- 
boscidian, the  Mastodon.  If  we  take  as  our  starting  point 
the  second  true  molar  of  one  of  the  Mastodons  (Tetralo- 
phodon)  Ave  find  its  crown  to  be  made  up  of  four  sti'ongly 
pronounced  transverse  ridges,  the  summits  of  which  are 
made  up  of  rounded  eminences  (whence  the  name  Mastodon, 


(')  Second  ujiper  molar  of  IMastudon  (longirostri.s),  from  Falconer. 
About  one-eighth  natural  size.  The  four  transverse  ridges,  h,  c,  d,  c,  are 
seen  to  be,  to  some  extent,  divided  into  outer  and  inner  divisions  by  a 
longitudinal  cleft,  much  less  deep  than  the  transverse  indentation.  At 
the  front  there  is  a  slight  elevation  of  cingulum  into  a  "  talon  "  (a),  and  a 
similar  one  at  the  back  of  the  tooth  ;  by  its  further  elevation  additional 
ridges  or  cusjis  would  be  formed. 


THE    TEETH    OF   MAMMALS.  295 


from  /Aaaros,  ix  nipple).  The  three  transverse  ridges  coalesce 
at  their  bases,  and  the  crown  is  siipportcd  upon  a  number 
of  roots  corresjionding  to  the  ridges. 

If  we  take  the  next  tooth,  or  the  third  true  molar,  the 
general  character  remains  the  same,  save  that  there  are  five 
ridges,  and  indications  of  as  many  roots ;  still  the  general 
correspondence  of  the  ridges  with  the  cusps  of  less  aberrant 
teeth  is  obvious. 

The  crown  is  coated  by  enamel,  over  which  there  is  a 
thin  layer  of  cement,  which  does  not  fill  up  the  whole 
interval  between  the  ridges. 

Thus  the  tooth  is  not  a  very  aberrant  one ;  it  is  obviously 
nothing  more  than  a  tooth  in  which  the  somewhat  numerous 
cusps  are  connected  bj-  transverse  ridges,  and  are  very  long 
and  strongly  pronounced. 

To  convert  the    tooth  of   a   mastodon    into    that    of   an 


elephant,  we  should  have  to  multiply  the  number  of  ridges, 
to  further  increase  their  depth,  to  fill  up  solidly  the  inter- 
spaces between  them  with  cementum,  and  to  stunt  the 
roots.  The  completed  tooth  of  an  elephant  is  a  squarish 
or  rather  oblong  mass,  from  the  base  of  which  sjDring  con- 
tracted and  stunted  roots.  It  consists  of  a  common  pulp 
cavity,   small  in  proportion  to  the  bulk  of  the  tooth,  and 

(')  Molar  tooth  of  an  Asiatic  Eleiihant,  sliowiiig  the  transverse  plates  of 
Jentiue  borJereil  by  enamel. 


296  A    MANUAL    OF   DENTAL    ANATOMY. 


deep  down  in  the  mass,  from  which  many  thin  laminee  are 
sent  up  towards  the  surface,  each  consisting  of  an  oblong 
area  of  dentine  enclosed  by  enamel ;  and  the  interspaces  of 
these  exaggerated  cusps  are  solidly  lillcd  in  by  cementum. 

Between  the  Mastodon  and  the  Indian  Elej)bant  are  a 
number  of  transitional  forms  in  which  we  are  able  to  trace 
the  gradual  modification  of  the  not  excessively  aberrant 
tooth  of  the  Mastodon  into  the  very  peculiar  huge  molar 
of  the  Indian  Elephant. 

The  numerous  transverse  })lates  of  the  elephant's  grinder."* 
are  united  by  dentine  at  their  bases,  and  a  common  pulp 
cavity  and  truncate  roots  are  formed ;  but  in  this  last 
respect  the  molar  teeth  of  the  ca})ybara  depart  still  farther 
from    the    ordinary   type,   for  Ijcing    molars    of   persistent 

Fig.  125  ('). 


growth,  their  numerous  transverse  plates  of  dentine  and 
enamel  do  not  become  continuous,  and  there  is  no  common 
pulp  cavity.  It  is  as  though  in  an  elephant's  grinder  the 
plates,  which  are  for  a  long  time  distinct,  never  coalesced, 
but  continued  to  grow  on  separately,  being  united  with  their 
fellows  by  cementum  only. 

It  has  been  suggested  (J.  A.  ilyder,  Proc.  Acad.  Nat. 
Sciences,  Philadelphia,  1878),  that  the  pattern  of  the  molar 
teeth  of  herbivora  is  the  result  of  the  extent  and  direction 
of  the  excursions  of  the  mandible  when  it  is  in  use,  and  so 
depends  upon  the  form  of  the  glenoid  cavity  and  of  the 
condyle,  and  that   hence  the  greatest  modification  is  to  be 

i})  Molar  of  CajDybara,  sliowing  the  transverse  plates  of  dentiue  and 
enamel  united  to  one  another  by  cementum. 


THE    TEETH    OF   MAMMALS.  297 

found  nearest  to  the  articulation,  where  the  greatest  force 
is  exei'ted. 

Thus  "bunodont"  animals,  z'.e.  those  that  have  rounded 
conical  cusps  ujjon  their  short-rooted  teeth,  have  a  cylindrical 
condyle  ;  selenodouts,  or  those  with  crescentic  ridges  on  the 
molars,  have  a  condyle  which  is  expanded  and  plane,  while 
lophodonts,  or  those  with  transversely  ridged  teeth,  have  a 
globular  condyle. 

This  correspondence  pointed  out  between  the  condyle,  the 
movements  of  the  jaw,  and  the  form  of  the  teeth  does  exist, 
but  it  is  less  easy  to  see  how  it  is  brought  about.  The 
simple  mechanical  explanation  that  the  teeth  are  drawn  out 
into  these  forms,  hardly  conveys  much  information,  seeing 
that  the  tooth  before  it  is  subjected  to  these  influences,  is 
quite  finished,  and  its  form,  such  as  it  is,  is  unalterable  : 
while  to  effect  an  alteration  in  the  form  of  a  masticating 
surface  an  influence  must  be  brought  to  bear  iipon  the 
tooth  germs  at  an  exceedingly  early  period.  It  might  with 
equal  justice  be  said  that  the  crown  of  the  tooth  being 
foi'med  thus  had  influenced  the  excursions  of  the  jaw,  and 
so  modified  the  condyle. 


THE    JIILK    DEXTITIOX. 


Some  thirty  years  ago  Professor  Owen  called  attention  tc> 
the  fact  that  those  mammals  in  whom  thd  teeth  situated  in 
different  parts  of  the  mouth  were  alike  in  form  (homodonts), 
developed  only  one  set  of  teeth,  and  to  indicate  this  charac- 
teristic he  proposed  for  them  the  term  "  monophyodonts." 
Those,  which,  on  the  contrary,  had  teeth  of  different  size 
and  form  in  various  parts  of  the  mouth  (heterodonts),  de- 
veloped two  sets  of  teeth  ;  a  "  milk  "  set,  which  was  dis- 
placed by  a  permanent  set,  and  this  peculiarity  he  expressed 
by  the  term  "  diphyodonts."  As  originally  set  forth,  the 
terms  homodont  and  monophyodont  were  interchangeable. 


A    MAXUAL    OF   DENTAL    AXATOMY. 


for  they  designated  the  same  groups  of  animals  ;  in  the 
same  way  heterodont  was  an  equivalent  for  diphyodont. 

But  although  this  is  true  of  a  large  number  of  animals,  it 
is  not  true  of  all,  and  it  becomes  necessary  to  note  some  of 
the  exceptions. 

The  nine-banded  armadillo  (Tatusia  puba)  is  a  true  homo- 
dont :  its  teeth  are  all  very  nearly  alike,  they  are  simple  in 
form,  and  they  grow  from  persistent  pulps.  Yet  it  has 
been  shown  by  Rapp,  Gervais,  and  Professor  Flower,  to 
have  a  well  developed  set  of  milk  teeth,  retained  until  the 
animal  is  of  nearly  full  size. 

Thus  it  is  a  true  diphyodont,  at  the  same  time  that  it  is 
a  true  homodont  mammal.  But  no  milk  dentition  has  been 
observed  in  the  sloths,  uor  indeed  at  present  has  it  been 
seen  in  any  other  armadillo  (except  the  doubtfully  distinct 
T.  Klapperi)  ;  nor  have  milk  teeth  been  found  in  any  ceta- 
cean, so  that  the  rest  of  the  homodont  animals  are,  so  far  as 
we  know,  really  monophyodont. 

Nor  is  it  absolutely  true  that  monopliyodonts  are  all 
homodont :  thus  the  rudimentary  teeth  of  balfcnoptera  are 
heterodont  (see  p.  311). 

Upon  the  whole,  our  information  respecting  the  "  milk  " 
or  deciduous  dentition  is  defective ;  but  much  light  has 
been  thrown  upon  the  subject  by  the  investigations  of 
Professor  Flower  (Journal  of  Anatomy  and  Physiology, 
1869,  and  Transactions  Odontological  Society,  1871),  of 
whose  papers  I  have  made  free  use  in  this  chapter. 

The  perpetual  replacement  of  teeth  lost,  or  shed  in  regular 
course,  which  characterises  the  dentition  offish  and  reptiles, 
finds  no  parallel  in  the  case  of  mammals,  none  of  whom 
develop  more  than  two  sets  of  teeth. 

Just  as  homodont  mammals  as  a  rule  develop  but  one  set 
of  teeth,  so  heterodont  mammals  as  a  rule  develop  two  sets 
of  teeth,  though  exceptions  to  this  rule  may  be  found. 

The  deciduous  or  milk  set  of  teeth  may  be  of  any  degree 


THE    TEETH   OF   MAMMALS.  299 

of  completeness  ;  the  milk  teeth  in  man  answer  the  require- 
ments of  the  child  up  to  the  age  of  seven  years,  and  in  the 
Ungulata  they  commonly  remain  until  the  animal  has 
assumed  its  adult  proportions.  On  the  other  hand,  in 
many  "  diphyodont  "  animals  the  milk  teeth  disapjiear  very 
early  indeed,  as  in  the  mole  (see  Fig.  117)  ;  whilst  there  are 
many  instances  of  the  milk  teeth  being  absorbed  in  utero. 
fSo  that  in  the  extent  to  which  the  milk  teeth  are  developed, 
the  greatest  variability  is  found  to  exist. 

A  perfectly  typical  milk  dentition  represents,  upon  a 
reduced  scale,  the  adult  dentition  of  the  animal,  with  the 
exception  only  that  sexual  differences  are  but  feelily  marked, 
if  indeed  the}-  are  at  all  present. 

Thus,  as  a  general  rule,  the  hindmost  of  the  milk  teeth 
bear  more  resemblance  to  the  true  molars  which  come  up 
behind  them,  than  they  do  to  the  premolars  which  come  up 
from  below  to  displace  them,  which  latter  are  generallj^  of 
simpler  form. 

In  what  may  be  termed  the  normal  arrangement,  each 
tooth  of  the  milk  series  is  vertically  displaced  b}^  a  tooth  of 
the  permanent  series ;  but  plenty  of  examples  may  be 
found  of  particular  milk  teeth  which  have  no  successors, 
and,  on  the  contrary,  individual  permanent  teeth  which 
have  never  had  a  deciduous  predecessor. 

It  has  already  l^een  mentioned  that  amongst  homodonts 
no  succession  of  teeth  has  been  observed  in  the  Cetacea, 
nor  in  any  other  of  the  Edentata,  save  the  armadillo ; 
amongst  heterodonts  there  are  several  Rodents  which  have 
no  deciduous  teeth,  e.g.,  the  rat ;  the  dugong  has  probably 
deciduous  incisors,  but  no  other  milk  teeth  ;  the  elephant 
has  no  vertical  succession,  save  in  the  incisors. 

Among  Marsupials,  which  are  true  heterodonts,  there  is 
only  one  milk  molar  on  each  side  in  each  jaw;  this  is  always 
displaced  by  the  third  or  last  premolar ;  but  the  milk  tooth 
varies  in  the  extent  to   which  it   is  developed  from  being 


A    MANUAL    OF   DENTAL    ANATOMY. 


rudimentary  in  Thylacinus,  probably  absent   altogether  in 

Dasyurus,  and  Phascolarctus,  to  being  a  large  tooth  retained 

in  full  use  till  the  animal  is  nearly  full  grown  in  Hypsiprymnus. 

Within  the  group  Carnivora,  the  dog  and  many  others 

Fig.  126  (i). 


Fiti.  127  (2). 


have  a  thoroughly  well  developed  set  of  milk  teeth,  which 
do  service  for  some  time  ;  in  the  l)ear  the  milk  teeth  are 
relatively  smaller,  and  are  shed  very  early ;  in  the  seal  the 
milk  teeth  arc  rudimentary,  functionless,  and  are  absorbed 
before  birth,  so  that  in  the  specimen  figured  the  milk  incisors 
had  already  disappeared  (see  Fig.  127). 

Q)  Permaneut  and  milk  dentitions  of  a  Dug  ;  tlic  iatter  was  well  de- 
veloped.    Nat.  size. 

(-)  Permanent  and  milk  dentition  of  a  seal  (Plioca  Greeulandia). 
Nat.  size. 


THE    TEETH    OF  MAMMALS. 


301 


In  the  elephant  seal  the  milk  teeth  are  yet  more  rudi- 
mentary, and  the  difference  between  its  dentition  and  that 
of  the  monophyodont  homodont  cetacean  (Grampus)  is  not 


Fig.  128  ('). 


■5  ncd  P.IZO 


Fir..  129  (-) 


iial  size 


gi-eat ;  an  observation  which  is  the  more  interesting,  inas- 
much as  this  seal  in  other  characters  than  its  teeth  ap- 
proaches towards  the  cetacean  group.  From  these  facts, 
which  are  well  indicated  in  the  accompanying  figures,  Pro- 
fessor Flower  arg-ues  that   the  permanent  set  of  teeth   of 


(')  Permanent  aiul  milk  ilentition  of  an  Elephant  Seal  (Cystophora 
proboscidea). 

(-)  Teeth  of  the  truly  moiioiihyoilal  (jrampiis  (Orca  capensis).  (These 
four  figures  are  copied  from  Prof.  Flower's  paper). 


302  A    MANUAL    OF   DENTAL    ANATOMY. 


diphyodonts  correspond  to  the  single  set  of  monophyodonts, 
so  that  the  milk  dentition,  when  it  exists  at  all,  is  some- 
thing superadded. 

Whether  this  1)e  so  is  a  question  difficult  to  determine ; 
from  the  facts  advanced  by  Professor  Flower,  while  they 
stood  alone,  most  people  would,  with  little  hesitation,  concur 
Avith  his  conclusions ;  but  the  history  of  the  development  of 
the  teeth  interposes  a  fresh  difficidty. 

The  tooth  germ  of  the  milk  tooth  is  first  f(jrmed,  and  the 
tooth  germ  of  the  permanent  tooth  is  derived  from  a  portion 
(the  neck  of  the  enamel  germ)  of  the  formative  organ  of  the 
milk  tooth  (see  Fig.  67).  Again,  in  most  of  those  animals 
in  which  there  is  an  endless  succession  of  teeth,  such  as  the 
snake,  the  newt,  or  the  shark,  each  successive  tooth  germ  is 
derived  from  a  similar  part  of  its  predecessor,  the  natural 
inference  from  which  would  be  that  the  permanent  set, 
being  derived  from  the  other,  was  the  thing  added  in  the 
diphyodonts. 

The  question  cannot  be  finally  settled  imtil  we  know  more 
of  the  development  of  the  teeth  of  the  monophyodont  cetacea : 
thus  it  might  turn  out  that  in  them  also  there  ai-e  abortive 
germs  of  milk  teeth  formed,  which  do  not  go  on  so  far  as 
calcification,  but  which  do  bud  off,  as  it  were,  germs  or  per- 
manent teeth ;  if  such  should  prove  to  be  the  case,  this 
would  bring  their  teeth  into  close  correspondence  with  those 
of  the  elephant  seal. 

The  investigation  of  these  questions  is  further  complicated 
by  the  fact  that  there  are  quite  numerous  instances  of  "  per- 
manent "  teeth,  that  is  teeth  unquestionably  belonging  to 
the  second  set,  which  are  shed  off  early,  and  do  not  remain 
in  place  through  the  lifetime  of  the  animal ;  an  example  of 
this  is  to  be  found  in  the  Wart  Hog  (Phacochserus),  which 
loses  successively  all  its  premolars  and  the  first  and  second 
true  molars,  the  last  true  molar  alone  being  truly  persistent. 

Sometimes  nothing  but  a  careful  comparison  of  the  teeth 


THE    TEETH    OF   JL-LUMALS.  303 


of  allied  creatures  will  eiial)le  us  to  decide  whether  a  parti- 
cular tooth  is  to  be  referred  to  the  milk  or  to  the  permanent 
series  ;  as  occasionally  teeth  of  the  latter  set  are  cut  very 
early,  at  a  time  when  the  milk  teeth  are  all  in  place,  and 
are  shed  during  adult  life.  Professor  Flower  gives  as  an 
example  of  this,  the  first  premolar  of  the  hippopotamus. 


CuviER.     Dents  des  Mammiferes. 

De  Blaixville.     Osteographie.     1839 — lSr)4. 

Owen.     Odontography.     184.5. 

G-IEBEL.     Odontographie.     18.i.>. 

Flower.     Lectures  on  Odontology  (British  Med.  Journal,  187 1). 


CHAPTER    IX. 

THE   TEETH   OF  MONOTREMATA,    EDENTATA,    AND   CETACEA. 

:monotremata. 

The  Echidna,  or  Spiny  Ant-eater  has  no  teeth  whatever, 
and  the  strange  Ornithorhyncus  (duck-ljilled  Platypus)  is 
also  destitute  of  true  calcified  teeth. 

In  the  place  of  teeth  its  flattened  hill  is  furnished  with 
eight  horny  plates,  two  on  each  side  of  each  jaw.  We 
may  therefore  pass  at  once  to  the  orders  Edentata  and 
Cetacea,  which  it  is  convenient  to  take  first,  as  their 
dentitions  are  of  that  simple  form  designated  by  the  term 
"Homodont." 

THE    TEETH    OF    EDENTATA    (BrUTa). 

Sloths,  Armadillos,  Ant-eaters. 

The  term  Edentata  was  applied  to  the  animals  of  this 
order  to  indicate  the  absence  of  incisors  (teeth  in  the  inter- 
maxillary bone)  :  though  this  is  true  of  most  of  them,  a  few 
have  some  upper  incisors,  but  the  central  incisors  are  in  all 
cases  wanting. 

Some  of  them  are  quite  edentulous ;  this  is  the  case  in 
the  Mutica,  or  South  American  Ant-eaters  (Myrmecophaga 
and  Cyclothurus),  in  which  the  excessively  elongated  jaws 
cannot  be  separated  to  any  considerable  extent,  the  mouth 
being  a  small  slit  at  the  end  of  the  elongated  muzzle. 
Food  is  taken  in  by  the  protrusion  of  an  excessively  long, 
whip-like  tongue,  which  is   covered  by  the  viscid  secretion 


THE    TEETH    OF   EDEXTATA.  305 

of  the  great  sub-maxillary  glands,  and  is  wielded  with 
much  dexterity.  The  Mauis,  or  Scaly  Ant-eater  is  also 
edentulous. 

The  Edentata  belong  to  the  monoj^hyodont  or  homodont 
section  of  Mammalia ;  but,  in  some,  certain  teeth  are  more 
largely  developed  than  others,  so  that  we  have  teeth  which 
might  be  termed  canines  ;  and  it  has  already  been  mentioned 
that  one  armadillo,  at  all  events,  is  diphyodont. 

The  teeth  are  of  simple  form,  and  do  not  in  any  marked 
degree  differ  in  the  different  parts  of  the  mouth,  except 
only  by  their  size  (to  this  the  canine-like  tooth  of  the  two- 
toed  sloth  is  an  exception).  They  are  all  of  persistent 
growth,  and  therefore  no  division  of  parts  into  crown,  neck, 
and  root  is  possible  :  they  consist  generally  of  dentine  and 
cement,  with  sometimes  the  addition  of  vaso-dentine,  into 
which  latter  tissue  the  central  axis  of  the  pulp  is  converted ; 
while  in  some  members  of  the  order  other  peculiarities  of 
structure  exist :  thus  in  the  Orycteropus  (Cape  Ant-eater), 
dentine  like  that  of  Myliobates  is  found ;  and  in  the  Megathe- 
rium hard  dentine,  a  peculiar  vaso-dentine,  and  richly 
vascular  cementum  co-exist  (see  Fig.  4.3). 

I  am  not  aware  that  enamel  has  been  seen  upon  the  teeth 
of  any  Edentate  animal,  but  I  found  some  years  ago  that 
the  tooth  germs  of  the  nine-banded  armadillo  were  provided 
with  enamel  organs ;  this,  however,  proves  nothing,  for 
(Philos.  Trans.,  1876)  I  believe  the  presence  of  enamel 
organs  to  be  uniA-ersal  and  quite  independent  of  any  after 
formation  of  enamel. 

The  teeth  of  the  nine-banded  armadillo  (T.  peba),  will 
serve  to  illustrate  the  character  of  the  dentition  of  the 
class.  They  are  seven  in  number  on  each  side  of  the  jaw,  of 
roundish  form  on  section,  and  those  of  the  upper  and  lower 
jaws  alternate,  so  that  by  wear  they  come  to  terminate  in 
wedge-shaped   grinding   surfaces  :   before   they   are   at   all 

X 


306  A    MANUAL    OF   DENTAL    ANATOMY. 


worn  they  are  bilobed,  as  may  be  seen  in  sections  of  the  tooth 
germs. 

In  the  accompanying  figure  the  milk  teeth  are  represented, 
and  beneath  them  their  permanent  successors  :  the  divari- 
cated bases  of  the  milk  teeth  are  due  to  the  absorption  set 
up   by   the    approach  of  their  successors,  and  not  to  the 


X2  I\.S. 


formation  of  any  definite  roots.  Successional  teeth  have 
been  detected  in  this  armadillo  only  (except  also  in  T, 
kappleri,  which  is  perhaps  a  mere  variety) ;  but  material 
does  not  exist  in  our  museums  which  would  enable  us  to 
positively  deny  their  occm-rence  in  other  forms. 

Professor  Flower  has  failed  to  discover  any  succession  of 
teeth  in  the  sloths,  and  I  have  myself,  through  the  kindness'of 
the  late  Professor  Garrod,  examined  microscopically  the  jaws 
of  a  foetal  Choloepus,  in  which  the  teeth  were  but  little 
calcified,  and  failed  to  detect  any  indication  of  a  second  set 
of  tooth  germs.  The  probability  is,  therefore,  that  they  are 
truly  Monophyodont. 

In  the  armadillos  the  teeth  are  always  of  simple  form  and 
about  thirty-two  in  number,  except  in  Priodon,  which  has  as 
many  as  a  hundred  teeth,  a  number  altogether  exceptional 
among  mammals. 

Sloths  have  fewer  teeth  than  armadillos,  and  these  softer 
in  character,  the  axis  of  vaso-dentine  entering  more  largely 

(1)  Lower  jaw  of  a  young  Armadillo  (Tatusia  peba),  showing  the  milk- 
teeth  (a)  in  place,  and  their  successors  (h)  beneath  them.  From  a  specimen 
in  the  Museum  of  the  Royal  College  of  Surgeons. 


THE    TEETH    OF    CETACEA.  307 

into  their  composition,  and  foimiug  as  much  as  half  the 
biilk  of  the  tooth. 

The  Orycteropus,  or  Cape  Ant-eater,  the  pecu]iarities  of 
■whose  teeth  have  ah-eady  been  alluded  to,  has  about  twenty- 
six  teeth  in  all ;  the  true  ant-eaters  are  edentulous.  The 
teeth  of  some  of  the  gigantic  extinct  Edentates  -were  a  little 
more  complex  in  foi'm  and  structure  ;  thus  the  teeth  of  the 
Glyptodon  were  divided  by  longitudinal  grooves,  which  in 
section  rendered  it  trilobed  ;  and  the  teeth  of  the  Mega- 
therium were  likewise  marked  by  a  longitudinal  furrow. 

In  their  persistent  growth,  uniformity  of  shape,  and 
absence  from  the  inter-maxiUary  bone,  they  strictly  con- 
formed with  the  teeth  of  recent  Edentata. 


THE    TEETH    OF    CETACEA. 


No  cetacean  is  known  to  develop  more  than  one  set  of 
teeth,  and  these,  when  present  in  any  considerable  numbers, 
closely  resemble  one  another  in  form. 

They  are  usually  composed  of  hai'd  dentine,  with  an  in- 
vestment of  cement ;  after  the  attainment  of  the  full 
dimensions  of  the  tooth  what  remains  of  the  pulp  is  very 
commonly  converted  into  secondary  dentine  ;  tips,  and  even 
entire  investments  <jf  enamel,  are  met  with  in  many  of  the 
order. 

The  dentine  of  many  Cetaceans,  e.  g.  of  the  sperui  whale, 
is  remarkable  for  the  very  numerous  interglobular  spaces 
which  it  contains  ;  these  are  clustered  in  concentric  rows, 
so  as  to  give  rise  to  the  appearance  of  contour  lines.  The 
cement  is  often  of  great  thickness,  and  the  lacunse  in  it  are 
very  abundant  ;  its  lamination  is  also  very  distinct. 

In  the  dolphin  the  teeth  are  very  numerous,  there  being 
about  200 ;  they   are  slender,    conical,   slightly  curved   in- 

X  2 


308 


A    MANUAL    OF   DENTAL    ANATOMY. 


wards,  and  sharply  pointed ;  as  they  interdigitate  with  one 
another  there  is  very  little  wear  upon  the  points,  which 
consequently  remain  quite  sharp.  The  largest  teeth  are 
those  situated  about  the  middle  of  the  dental  series. 

Fig.  131  (i). 


Many  variations  in  the  number  and  form  of  the  teeth 
are  met   with  ;  the  porpoise  has  not  more  than  half  the 


r^ 


TTiat"  sxze 


W\r\i\) 


■^ 


number  of  teeth  possessed  by  the  dolphin,  wliile  the  gram- 
pus has  still  fewer.  The  teeth  of  the  grampus  become 
worn  down  on  their  opposed  surfaces,  and  coincidently 
with  their  wearing  away  the  pulps  become  calcified.  In 
the  Oxford  museum  there  is  a  grampus  in  which,  owing  to 
a  distortion  of  the  lower  jaw,  the  teeth,  instead  of  inter- 
digitating,  became  exactly  opposed  to  one  another ;  the 
consequence  of  this  was  that  the  rate  of  wear  was  greatly 
increased,   and   the   pulp   cavities  were  opened  before  the 


(1)  Jaws  of  a  common  Dolpliin. 

(-)  Teetli  of  upper  jaw  of  a  Gramims  (after  Professor  Flower). 


THE   TEETH    OF    CETACEA.  309 


obliteration  of  the  pulps  by  calcification,'  so  that  the  pulps 
died  and  abscesses  around  the  teeth  had  resulted. 

In  the  sperm  whale  the  teeth  are  numerous  in  the  lower 
jaw,  but  in  the  upper  jaw  there  are  only  a  few  curved, 
stunted  teeth,  which  remain  bvu-ied  in  the  dense  gum.  The 
teeth  of  the  lower  jaw  are  retained  in  shallow  and  wide  de- 
pressions of  the  bone  by  a  dense  ligamentoiis  gum,  which, 
when  strijiped  away,  carries  the  teeth  with  it.  Every  inter- 
mediate stage  between  this  slight  implantation  and  the 
well-developed  stout  sockets  of  the  grampus,  is  met  with  in 
the  Cetacea. 

In  the  bottle-nosed  whale  (Hyperoodon  bidens)  the  only 
large  teeth  present  are  two  conical,  enamel-tipped  teeth 
(sometimes  four)  which  remain  more  or  less  completely 
embedded  within  the  gum,  near  to  the  front  of  the  lower 
jaw:  in  addition  to  these  there  are  12  or  13  very  small 
rudimentary  teeth  loose  in  the  gums  of  both  jaws.  (Esch- 
richt,  Lacepede.) 

In  the  narwal  (Alonodon  monoceros)  two  teeth  alone  per- 
sist, and  these  are  in  the  upper  jaw.  In  the  female  the 
dental  germs  become  calcified,  and  attain  to  a  length  of 
about  eight  inches,  but  they  remain  enclosed  within  the 
substance  of  the  bone,  and  their  pulp  cavities  speedily  fill 
up.  In  the  male,  one  tusk  (in  some  very  rare  instances 
both)  continues  to  grow  from  a  persistent  pulp  till  it  at- 
tains to  a  length  of  ten  or  twelve  feet,  and  a  diameter  of 
three  or  four  inches  at  its  base.  This  tusk  (the  left)  is 
quite  straight,  but  is  marked  by  spiral  grooves,  winding 
from  right  to  left.  It  is  curious  that  in  one  of  the  speci- 
mens, in  which  the  two  tusks  had  attained  to  equal  and 
considerable  length,  the  spirals  on  the  two  wound  in  the 
some  direction ;  that  is  to  say,  as  regards  the  sides  of  the 
head,  the  spirals  were  not  symmetrical  with  one  another. 

(1)  Trans.  Odonto.  Society,  1873.  When  I  published  this  paper  I  was 
not  aware  that  Eschricht  had  previously  published  a  similar  observation. 


310 


A    MANUAL    OF  DENTAL    ANATOMY. 


The  tusk  of  the  male  narwal  may  fairly  be  assumed  to 
serve  as  a  sexual  weapon,  but  little  is  known  of  the  habits 
of  the  animal. 

Fig.  133  Q). 


Professor  Turner  has  lately  noted  the  occurrence  of  two 
stunted  incisor  rudiments  in  a  foetal  narwal :  these  ob- 
viously repi-esent  a  second  pair  of  incisors,  and  attain  to  a 
length  of  half  an  inch,  but  are  irregular  in  form  ;  they  are 
situated  a  little  behind  the  jiair  of  teeth   which  attain  to 

(')  Cranium  of  Narwal  (Monodoii  monoceros).  a.  Stunted  tooth,  witt 
its  basal  pulp-cavity  obliterated,  h.  Long  tusk.  The  small  figure,  giving 
the  whole  length  of  the  tusk,  shows  the  proportion  which  it  bears  to  the 
vest  of  the  skull. 


THE    TEETH    OF   CETACEA. 


more  considerable  dimensions.     All  trace  of  this  second  pair 
of  incisors  is  lost  in  adult  skulls. 

The  Cetaceans  classed  together  as  Ziphoids  have  no  teeth 
in  the  upper  jaM',  and  in  the  lower  jaw  only  two  (in  a 
single  species  there  are  four)  teeth  which  attain  to  any- 
considerable  size,  though  other  rudimentary  teeth  have  been 
formed  in  the  dense  gum. 

The  structure  of  these  teeth  is  very  peculiar  :  a  tooth  of 
a  species  of  Ziphius  in  the  Oxford  University  museum, 
which  was  described  by  Professor  Ray  Lankestei",  consists  in 
great  part  of  cementum  and  osteo-dentine,  the  true  dentine 
being  merely  a  little  fragment  situated  at  the  top,  and  not 
forming  more  than  a  tenth  of  the  whole  bulk  of  the  tooth. 

Zipbius  Layardii  has  teeth  nearly  a  foot  long  projecting 
upwards  from  the  lower  jaw,  which  arch  towards  one 
another,  above  the  upper  jaw,  so  that  the}"  must  prevent 
the  mouth  from  being  opened  to  any  considerable  extent. 

The  whalebone  whales  are,  in  the  adult  condition,  des- 
titute of  teeth,  but  prior  to  birth  the  margins  of  both  upper 
and  lower  jaws  are  covered  with  a  series  of  nearly  globular 
rudimentary  teeth,  which  become  calcified,  but  are  speedily 
shed,  or,  rather,  absorbed. 

The  foetal  teeth  of  the  Baleenoptera  rostrata  have  been 
cai'cfully  described  by  M.  Julin  (Archives  de  Biologic,  1880), 
the  Bakenopte7'idce  having  been  previously  supposed  to  be 
without  rudimentary  teeth.  The  ramus  contained  41  tooth 
germs,  each  furnished  with  an  enamel  organ  and  dentine 
bulb,  with  a  slight  capsule ;  these  were  lodged  in  a  con- 
tinuous groove  in  the  bone  above  the  vessels,  thus  re- 
calling the  condition  of  the  parts  in  a  human  embryo  at  a 
certain  stage.  A  very  small  amount  of  calcification  takes 
place,  a  mere  film  of  dentine  being  formed  upon  the  dentine 
bulb.  But  what  is  very  remarkable  is  that  the  dentine 
bulbs  a,re  simple  near  the  front,  bifid  in  the  middle,  and 
trifid   at   the  back    of   the  mouth ;    in  other  words,  these 


312  A    MANUAL    OF  DENTAL   ANATOMY. 

rudimentary  teeth  seem  not  to  be  rudiments  of  a  homo- 
dont  dentition  as  might  have  been  expected,  but  of  a 
heterodont  dentition  ;  and  it  is  suggestive  of  a  resemblance 
to  such  forms  as  Squalodon,  an  extinct  cetacean,  peculiar  in 
having  heterodont  teeth.  At  all  events  it  seems  to  indicate 
that  the  homodont  dentition  of  Cetacea  is  a  case  of  degrada- 
tion from  ancestral  forms,  a  conclusion  likewise  pointed  to 
by  the  gradual  suppression  of  milk  dentitions  (see  p.  300). 

From  the  upper  jaw  of  an  adult  whalebone  whale  there 
hang  down  a  series  of  plates  of  baleen,  placed  transversely  to 
the  axis  of  the  mouth,  but  not  exactly  at  right  angles  to  it. 
The  principal  plates  do  not  extend  across  the  whole  width  of 
the  palate,  but  its  median  portion  is  occupied  by  subsidiary 
smaller  plates.  The  whalebone  plates  are  frayed  out  at 
their  edges,  so  as  to  be  fringed  with  stiff  hairs,  and  their 
fringed  edges  collectively  form  a  concave  roof  to  the  mouth, 
against  which  the  large  tongue  fits,  so  as  to  sweep  from  the 
fringes  whatever  they  may  have  entangled.  The  w^hale  in 
feeding  takes  in  enormous  mouthfuls  of  water  containing 
small  marine  mollusca ;  this  is  strained  through  the  baleen 
plates,  which  retain  the  Pteropods  and  other  small  crea- 
tures, while  the  water  is  expelled.  Then  the  tongue  sweeps 
the  entangled  food  from  the  fringe  of  the  baleen  plates  and 
it  is  swallowed.  Each  plate  consists  of  two  dense  but 
rather  brittle  lamiuse,  which  enclose  between  them  a  tissue 
composed  of  bodies  analogous  to  coarse  hairs.  By  the  pro- 
cess of  wear  the  brittle  containing  lamince  break  away, 
leaving  projecting  from  the  edge  the  more  elastic  central 
tissue,  in  the  form  of  stiff  hairs. 

Each  plate  is  developed  from  a  vascular  persistent  pulp, 
which  sends  out  an  immense  number  of  exceedingly  long- 
thread-like  processes,  which  penetrate  far  into  the  hard 
substance  of  the  plate.  Each  hair-like  fibre  has  within  its 
base  a  vascular  filament  or  papilla :  in  fact,  each  fibre  is 
nothing  more  than  an  accumulation  of  epidermic  cells,  con- 


THE    TEETH    OF    CETACEA.  313 


centrically  arranged  around  a  vascular  papilla,  the  latter 
being  enormously  elongated.  The  baleen  plate  is  composed 
mainly  of  these  fibres,  which  constitute  the  hairs  of  its  frayed- 
out  edge,  but  in  addition  to  this  there  are  layers  of  flat  cells 
binding  the  whole  together,  and  constituting  the  outer  or 
lamellar  portion.  As  has  been  pointed  out  by  Prof.  Turner 
(Proc.  Roy.  Soc.  Edinbm-gh,  1870),  the  whalebone  matrix 
having  been  produced  by  the  cornification  of  the  epithelial 
coverings  of  its  various  groups  of  papillte,  is  an  epithelial  or 
epiblastic  structure,  and  morphologically  corresponds  not 
with  the  dentine,  but  with  the  enamel  of  a  tooth. 

The  wdiole  whalebone  plate  and  the  vascular  ridges  and 
papillce  which  form  it  may  be  compared  to  the  strong  ridges 
upon  the  palates  of  certain  Herbivora,  an  analogy  which  is 
strengthened  by  the  study  of  the  mouth  of  young  whales 
prior  to  the  cornification  of  the  whalebone. 


CHAPTER  X. 

THE    TEETH    OP    UNGULATA. 

In  the  two  orders  just  considered,  the  Cetacea  and 
Edentata,  a  single  set  of  teeth  would  seem  to  be  the  rule, 
and  most  members  of  these  orders  are,  so  far  as  is  known, 
both  monophyodont  and  homodont.  But  in  all  orders  that 
remain  to  be  considered  a  Diphyodont  dentition,  the  milk  set 
varying  from  the  merest  rudiments  to  full  development,  Avill 
be  the  rule ;  and  being  diphyodont,  they  are  for  the  most 
part  heterodont,  that  is  to  say,  the  teeth  differ  from  one 
another,  and  we  can  distinguish  them  into  incisors,  canines, 
premolars,  and  molai's.  Hence  we  are  able  to  assign  to  them 
a  dental  formula,  and  an  extended  survey  of  mammalian 
forms  lends  strong  support  to  the  idea  that  the  typical  dental 
formula,  in  which  the  full  normal  mammalian  number  of 
teeth  is  present,  is 

.31  4         3 

1  —  c  —  rirm   —  m  —  =  4-1. 
3        1^43 

Very  many  have  less  than  this  full  number  :  only  a  few 
have  more  ;  and  it  is  not  a  little  interesting  to  find  that 
among  extinct  mammalia,  and  especially  among  extinct 
ungulata,  the  typical  dentition  was  more  often  present  than 
amongst  recent  animals.  Indeed  it  may  be  said  that  most 
mammals  of  the  Eocene  period  had  the  full  typical  mam- 
malian dentition. 


THE    TEETH    OF    VNGULATA.  315 

Ungulata,  or  hoofed  animals  are  grouped  thus  : — 

(i.)  Ai-tiodactyles,  or         )  Hippopotamus.  Pigs.  Anoplotherium.&c, 
even-toed  Ungulata  j  Cows,  Sheep.  Deer,  and  other  Ruminants. 

(ii.)  Perissodactyles.  or)  Horses.  Tapirs.  Rhinoceros.  Pateotherium. 
I  ngulata  with  au  / 
odd  number  of  toes  ) 

The  distinction  between  the  two  groups  is  strongly  marked,  if 
living  animals  alone  be  considered ;  but,  as  Professor  Huxley  has 
pointed  out.  increasing  knowledge  of  fossil  fonns  is  tending  to 
break  do^^-n  the  line  of  demarcation. 

The  recent  forms  bear,  in  all  probability,  but  a  very  small  pro- 
portion to  the  extinct  Ungulata.  of  which  our  knowledge  is  as  yet 
but  fragmentary  ;  though  the  discoveries  of  Professors  Marsh  and 
Cope  in  the  '■  mauvaises  teiTes  "  of  Wyoming  have  brought  to  light 
a  very  large  nmiiber  of  strange  and  interesting  ungulates  :  and  this 
fragmentary  condition  of  oui-  knowledge  makes  it  as  yet  impossible 
to  give  a  connected  account  of  the  dentition  of  ungulates,  seeing 
that  the  forms  known  to  us  are  only  isolated  and  often  widely 
separated  links  in  the  chain. 

The  Teeth  of  Perissodactyle  Ungulates.— Perisso- 
dactyle  (odd-toed)  Ungulates  are  far  less  numerous  than  the 
even-toed  section,  and  among  recent  animals  only  comprise 
the  Horse,  the  Rhinoceros,  the  Tapir,  and  their  allies.  Their 
premolars,  or  at  least  the  last  three  of  them,  are  eqiially 
complex  in  patteni  with  the  true  molars ;  and  canines,  tusk- 
like but  not  A'ery  large,  are  of  frequent  occurrence.  The 
lower  molai's  of  almost  all  perissodactyles  have  a  character- 
istic form,  their  grinding  surfaces  being  made  up  of  two 
ci'escentic  ridges. 

The  ungulate  animals  are  all  possessed  of  molar  teeth, 
which  are  kept  iu  an  efl&cient  state  of  roughness  by  the 
enamel  dipping  deeply  into  the  crowns  ;  by  the  cusps,  in 
fact,  being  of  very  great  depth.  It  consequently  happens 
that  after  the  immediate  apex  is  worn  away,  the  flattened 
working  face  of  the  tooth  is  mapped  out  into  definite 
patterns,  which,  on  account  of  the  light  thus  thrown  upon 
fossil  remains,  often  consisting  of  little  else  than  the  teeth, 
have  been  studied  with  great  care.     The  result  has  been  to 


316  A    MANUAL    OF   DENTAL    ANATOMY. 


establish  a  general  community  of  type,  so  that,  dissimilar 
as  they  at  first  sight  appear,  it  is  possible  to  derive  all,  or 
almost  all,  the  configurxitions  of  their  crowns  from  one  or 
two  comparatively  sim2Dle  pattei'ns.  But  odoutologists  are 
not  yet  agreed,  or  rather  do  not  yet  know  enough  of  the 
vast  number  of  extinct  Ungulates  which  there  is  reason  to 
believe  once  existed  (of  which  many  have  lately  been  dis- 
covered) to  decide  with  certainty  what  the  parent  pattern 
was. 

Bihinoceros. — It  is  difficult  to  assign  a  dental  formula  to 
this  genus,  as  the  incisors  are  variable  in  the  different 
species,  but  all  agree  in  the  absence  of  canines. 

.  2,      0        4        3 
1  — ]  c  —  p  —  m  — 

In  the  African  Rhinoceros,  in  which  the  adult  has  no 

incisors,  the  young  animal  has  eight  incisors  ;  other  species 

retain  the  incisors  through  life  ;  and  it  is  noteworthy  that 

2 
in  the  Indian  Rhinoceros,  which  has  i  — ,  the  outer  incisors 

in  the  upper  javv,  are,  as  is  usual,  the  ones  that  are  absent,  but 
in  the  lower  jaw  it  is  the  central  incisors  which  are  missing. 
The  first  premolar,  just  as  in  the  Horse,  is  small,  has  no 
milk  predecessor,  and  is  not  long  retained ;  the  other  pre- 
molars do  not  markedly  differ  from  the  true  molars.  The 
premolars  and  molar  teeth,  though  not  diflfering  much  in 
character,  increase  in  size  from  before  backwards.  The 
crowns  of  the  teeth  are  of  squarish  outline,  larger  on  their 
outer  than  their  inner  side,  and  are  implanted  by  four  roots. 
The  pattern  of  their  grinding  surfaces  is  very  characteristic; 
but  it  will  be  best  understood  by  first  digressing  to  say  a 
few  words  on  the  dentition  of  the  Tapir. 
Tapir. — The  dental  formula  is 

.3143 

o        l        6        o 


THE  TEETH  OF  PEEISSODACTYLE  UXGULATA.     317 

In  a  brief  survey,  like  that  to  which  the  present  work  is 
necessarily  confined,  it  will  suffice  to  mention  that  there  is 
no  great  peculiarity  about  the  incisors,  or  the  canines,  save 
that  the  lower  canine  ranges  with  the  lower  incisors ;  behind 
the  canine  comes  an  interval,  after  which  come  the  pre- 
molars and  molars,  which  are  interesting,  as  being  of  simpler 
pattern  than  those  of  most  Ungulates,  and  it  will  be  necessary 
to  very  briefly  allude  to  the  various  patterns  characteristic 
of  ungulate  teeth,  with  a  view  of  showing  how  they  may 
have  been  derived  the  one  fi'om  the  other. 

In  the  Tapir  four  cusps  are  traceable,  but  ridges  uniting 
the  two  anterior  and  the  two  posterior  cusps  are  strongly 
developed,  at  the  cost  of  the  antero-posterior  depression,  i.  e. 
of  one  of  the  arms  of  the  cross  which  separates  the  four  cusps 
in  other  quadricuspid  molars.  There  is  therefore  left  only  a 
deep  ti-ausverse  fissure  (hence  it  is  called  a  bilophodont 
tooth),  and  the  quadricuspid  form  is  disguised.  A  low  wall 
on  the  outside  of  the  tooth  connects  the  two  ridges. 

In  the  Hog  we  have  a  simple  fourcusped  molar,  with  a 


Fig. 

134(1 

). 

,   7nj 

7"", 

]l 

m^ 

i 

'  ^^^-:r' 

<w^ 

^. 

"> 

' '-  ■■":.•  ''■ 

1> 

~^KJ'-^ 

^vJ^ 

^'^ 

c 

d 

crucial  depression  separating  the  cusps  ;  in  the  Hippopot- 
amus the  same  pattena  is  repeated,  though  not  quite  so 
simply,  as  each  cusp  is  fluted  in  a  definite  manner. 

In  Rhinoceros  the  two  external  cusps  are  united  by  a 

(1)  Grinding  surfaces  of  upper  molar  series  of  a  Rhinoceros,  a.  Posterior 
sinus,  which  at  a'  has  become  an  island,  c.  Posterior  ridge,  d.  Anterior 
ridse. 


318  A    MANUAL    OF    DENTAL    ANATOMY. 


longitudinal  ridge,  possibly  the  ciugulum,  and  the  transverse 
ridges  become  oblique ;  consequently  the  valley  between 
the  ridges  c  and  d  is  also  oblique  in  direction,  and  a 
second  valley  "a"  behind  the  posterior  ridge  is  introduced 
(Fig.  134). 

The  simplicity  of  the  pattern  is  also  departed  from  by 
the  margins  of  the  ridges,  and  therefore  the  boundaries  of 
the  depressions,  being  waved  and  irregular. 

The  lower  molars  of  the  Rhinoceros  are  made  up  of  two 
crescentic  ridges,  one  in  front  of  the  other,  with  the  hollows 
turned  inwards.  It  is  less  obvious  how  this  pattern  is 
derived  from  that  of  the  Tapir,  but  it  may  be  that  the  trans- 
verse ridges  of  the  Tapir  type  of  tooth  may  have  become 
ciirved  and  crescentic,  so  that  the  original  outer  edge  of  the 
posterior  ridge  abuts  against  the  exterior  of  the  ridge  in 
front  of  it.  The  valleys  between  the  processes  of  enamel 
and  dentine  of  the  tooth  of  the  Rhinoceros,  termed  "  sinuses," 
are  not  filled  up  solidly  with  cementum.  The  more  complex 
pattern  which  characterises  the  molar  of  the  Horse  may  be 
derived  from  a  further  modification  of  the  Rhinoceros  molar. 

To  use  the  words  of  Professor  Huxley  :  "  Deepen  the 
valley,  increase  the  curvature  of  the  (outer)  wall  and  laminse 
(transverse  ridges),  give  the  latter  a  more  directly  backward 
slope ;  cause  them  to  develop  accessory  ridges  and  pillars ; 
and  the  upper  molar  of  the  Tapir  will  pass  thi'ough  the 
structure  of  that  of  the  Rhinoceros  to  that  of  the  Horse." 

By  a  further  increase  in  the  obliquity  of  the  ridges  and 
in  their  curvature  (c  and  d),  they  become  parallel  to  the 
external  or  antero-posterior  ridge  (wall),  and  bend  round 
until  they  again  touch  it,  thus  arching  round  and  completely 
encircling  the  sinuses  {a  and  the  space  between  c  and  d)  in 
the  Rhinoceros  tooth.  In  this  way  the  unsymmetrical  pattern 
of  the  Rhinoceros  tooth  may  be  supposed  to  become  trans- 
foi-med  into  the  comparatively  symmetrical  one  of  the  Horse 
or  of  the  ruminant. 


THE  TEETH  OF  PERISSODACTYLE  VXQULATA.       319 

The  outer  ridge  or  wall  is  in  the  upper  molar  of  the  horse 
doubly  bent,  the  concavities  looking  outwards.  The  trans- 
verse ridges  start  inwards  from  its  anterior  end  and  from  its 
middle,  and  they  ciu've  backwards  as  they  go  to  such  an 
extent  as  to  include  crescentic  spaces  (between  themselves 

Fig.  135  ('). 


and  the  outer  wall).  To  this  we  must  add  a  vertical  pillar, 
which  is  slightly  connected  with  the  posterior  end  of  each 
crescentic  edge  (this  pillar  is  in  Hippariou  quite  detached). 

The  lower  molars  of  the  horse  present  the  double  crescent, 
just  like  those  of  the  rhinoceros,  save  that  vertical  pillars 
are  attached  to  the  posterior  end  of  each  crescent,  thus 
slightly  complicating  the  pattern  of  the  worn  sxirface.  The 
interspaces  of  the  ridges  and  pillar  are  in  the  horse  solidly 
filled  in  with  cementum.  The  extinct  ancestors  of  the  horse 
have  the  molar  pattern  considerably  simphfied,  but  yet 
recognisable  as  being  built  up  on  the  same  model. 

But  in  an  elementary  handbook  such  as  this  it  will  only 
serve  to  perplex  the  reader  to  enter  into  a  discussion  of 
the  relative  probabilities  of  the  various  and  incompatible 
explanations  given  of  the  homologies  of  the  parts  of  the 
ungulate  molar  :  suffice  it  that  such  coiTespondences  do  exist, 
and  if  we  had  before  us  perfect  chains  of  molars  from  eveiT 

(1)  Ivlolar  tooth  of  a  Horse,  stowing  tlie  pattern  of  its  grinding  surface. 


A    MANUAL    OF   DENTAL    ANATOMY. 


ungulate  which  ever  lived,  there  would  be  no  doubt  as  to 
the  relationship  of  the  various  patterns  :  as  it  is,  we  are  em- 
barrassed by  the  lack  of  the  material,  which  leaves  gaps  too 
great  to  bridge  over  without  some  amount  of  speculation. 
As  it  is,  Professor  Flower  divides  the  principal  varieties 
(Phil.  Trans.,  1874,)  into  three  :— 

(i.)  That  in  which  the  outer  wall  is  feebly  develoi:)ed, 
and  transverse  ridges  become  the  prominent  features,  as  in 
the  tapir. 

(ii.)  That  in  which  the  outer  wall  is  gi-eatly  developed 
and  more  or  less  smooth,  the  transverse  ridges  being  oblique, 
as  in  the  rhinoceros. 

(iii.)  That  in  which  the  outer  surface  and  edge  of  the 
outer  wall  is  zigzagged,  or  bicrescentic,  as  in  the  horse  and 
palceotherium. 

Equus. — The  horse  is  furnished  with  the  full  mammalian 
number  of  teeth,  the  dental  formula  being — 

.3      1       4      .3 

The  canines,  however,  are  rudimentary  in  the  female,  whilst 
in  the  male  they  are  well  developed  (in  the  gelding  they  are 
of  the  same  size  as  in  the  entire  horse) ;  and  the  first  pre- 
molai',  which  has  no  predecessor,  is  also  rudimentary,  and  is 
lost  early.  A  considerable  interval  exists  between  the 
incisors  and  the  premolars  and  molars,  which  latter  are  very 
similar  to  one  another,  both  in  shape,  size,  and  in  the 
pattern  of  the  grinding  surface. 

The  incisors  of  the  horse  are  large,  strong  teeth,  set  in 
close  contact  with  one  another ;  the  teeth  of  the  upper  and 
lower  jaws  meet  with  an  "  edge  to  edge  bite,"  an  arrange- 
ment which,  while  it  is  eminently  adapted  for  grazing,  leads 
to  great  wearing  down  of  the  crowns.  An  incisor  of  a  horse 
or  other  animal  of  the  genus  may  be  at  once  recognized  by 
that  peculiarity  which  is  known  as  the  ''mark.  " 


THE  TEETH  OF  PERISSODACTYLE  UXGULATA.     321 


From  the  grinding  surface  of  the  crown  there  dips  in  a 
fold  of  enamel,  forming  a  cid  cle  sac.  As  this  pit  does 
not  extend  the  whole  depth  of  the  crown,  and  the  in- 
cisors of  a  horse  are  submitted  to  severe  wear,  the  fold 
eventually  gets  worn  away  entirely,  and  the  worn  surface 
of  the  dentine  presents  no  great  peculiarity.  But  as  this 
wearing  down  of  the  crown  takes  places  at  something  like  a 
regular  rate,  horse  dealers  are  enabled  to  judge  of  a  horse's 
age  by  the  appearance  of  the  mark  upon  the  diffei-ent  incisors. 
The  "  mark "  exists  in  Hippariou,  but  not  in  the  earlier 
progenitors  of  the  horse. 


FiQ.  136  (1). 


FiG.137('). 


A  horse  attains  to  its  adult  dentition  very  slowly ;  the 
first  permanent  incisors  appear  about  the  end  of  the  third 
year,  and  the  other  two  pairs  follow  at  intervals  of  about 


(1)  Incisors  of  the  Horse,  showing  the  marks  at  various  stages  of  wear. 


322 


A    MANUAL    OF   DENTAL    ANATOMY. 


six  mouths.  As  the  rate  of  wear  is  equal,  the  mark  gets 
worn  out  soonest  upon  the  central  incisors  (about  the  sixth 
year) ;  in  the  middle  incisors  next  (about  the  seventh),  while 
it  has  totally  disappeared  by  about  the  eighth  year. 

After  the  "  mark  "  is  worn  away  the  centre  of  the  tooth 
is  marked  by  a  difference  of  colour,  due  to  the  presence  of 
secondary  dentine,  into  which  the  remains  of  the  pulp  has 
been  converted. 

Fig.  138  ('). 


The  molars  of  the  horse  are  remarkable  for  their  great 
length ;  they  do  not  grow  from  persistent  pulps,  but  never- 
theless they  do  go  on  growing  until  a  great  length  of  crown 
of  uniform  diameter  is  made,  subsequently  to  which  the 
short  and  irregular  roots  are  formed.  As  the  upper  working 
surface  of  the  crown  becomes  worn,  the  tooth  rises  bodily  in 
its  socket,  and  when  by  an  accident  its  antagonist  has  been 
lost,  it  rises  far  above  the  level  of  its  neighbours.  This 
elevation  of  the  tooth  takes  place  quite  independently  of 


(1)  Side  view  of  the  dentition  of  a  Stallion.  At  a  short  interval  behind 
the  incisors  are  seen  the  canines  ;  then,  after  a  considerable  interval,  the 
premolar  and  molar  series. 


THE    TEETH  OF  PERISSODACTYLE  VNGVLATA.     323 

growth  from  a  persistent  pulp,  and,  in  fact,  happens  after 
the  formation  of  its  roots. 

The  pattern  of  the  horse's  molar  has  been  already  de- 
scribed ;  it  should  be  added  that  the  last  molar  differs  from 
the  rest  in  its  posterior  moiety  being  less  developed  than  in 
the  other  teeth. 

As  each  ridge  and  each  pillar  of  the  tooth  consists  of  den- 
tine bordered  by  enamel,  and  the  arrangement  of  the  ridges 
and  pillars  is  complex  ;  as,  moreover,  cementum  fills  up  the 
interspaces,  it  will  be  obvious  that  an  efficient  rough  grinding 
surface  will  be  preserved  by  the  unequal  wear  of  the  several 
tissues. 

When  a  bit  is  put  into  a  horse's  mouth  it  rests  in  the 
intei-val,  or  diastema,  which  exist  between  the  incisors  and 
the  commencement  of  the  molar  series,  and  the  great  con- 
venience of  the  existence  of  such  a  space  has  led  many 
authors  to  assume  that  the  horse  was  moulded  in  accordance 
with  man's  special  requirements,  so  that  it  might  be  suited 
for  its  subserviency  to  his  wants. 

But  the  wide  diastema  appeared  in  the  remote  ancestors 
of  the  horse  long  ages  before  man's  appearance  on  the  earth, 
and  the  advocates  of  this  theory  of  design  would,  as  Professor 
Huxley  suggests,  have  to  tell  us  what  manner  of  animal  rode 
the  Hipparion. 

The  milk  teeth  of  all  the  Ungulata  are  very  complete, 
and  are  retained  late  ;  they  resemble  the  permanent  teeth 
in  general  character,  but  the  canines  of  the  horse,  as  might 
have  been  expected,  their  greater  development  in  the  male 
being  a  sexual  character,  are  rudimentary  in  the  milk 
dentition. 

To  the  Perissodactyle  Ungulates  which  are  specially  inte- 
resting on  account  of  their  dentition,  must  be  added  Homa- 
lodontotherium,  a  tertiary  mammal,  the  remains  of  which  were 
described  by  Professor  Flower  (Phil.  Trans.,  1874). 

It    had    highly    generalised    characters  ;    its    teeth    were 

Y  2 


324  A    MANUAL    OF   DENTAL    ANATOMY. 


arranged  without  any  diastema,  and  the  transition  in  form 
from  the  front  to  the  back  of  the  mouth  was  exceedingly 
gradual,  so  that  no  tooth  diifered  much  ft-om  those  on  either 
side  of  it.  Taking  the  pattern  of  its  molar  teeth  alone  into 
account,  it  would  have  been  without  hesitation  declared  to 
be  very  nearly  allied  to  rhinoceros,  on  which  type  they 
are  formed,  but  the  resemblance  fails  in  the  canine  and 
incisor  region,  and  it  must  be  considered  to  be  one  of  those 
generalised  ij\)Q'&  related  to  rhinoceros,  to  Hyracodon  and 
perhaps  connecting  them  with  such  aberrant  forms  as 
Toxodon, 

The  largest  of  Perissodactyles  equalled  the  elephant  in 
size,  and  have  been  named  by  Prof.  Marsh  Brontotheridce. 
The  dental  formula  was 

.21  43 

1    ^    c    -    pm      -  m  -  . 

The  incisors  were  small  and  sometimes  deciduous,  and  the 
canines  short  and  stout,  the  lower  being  the  more  conspicuous 
owing  to  its  being  separated  by  a  slight  diastema  from  the 
premolars,  which  is  not  the  case  in  the  upper  jaw. 

The  premolars  in  both  jaws  increase  in  size  from  before 
backwards,  and  do  not  differ  from  the  molars  next  them.  In 
the  lower  jaw  the  premolars  and  molars  all  consist  of  two 
crescents,  save  the  last,  which  have  three  crescentic  cusps. 
The  molar  teeth  stand  apart  from  those  of  any  recent  peris- 
sodactyles in  their  huge  size,  the  squarish  last  upper  molar, 
for  example,  measuring  four  inches  antero-posteriorly  and 
more  than  three  transversely  (Prof.  Marsh,  American  Journal 
of  Science  and  Arts,  1876). 


THE  TEETH  OF  ARTIODACTYLE  UNGULATA.       325 


THE    TEETH    OF    ARTIODACTYLE    UNGULATA. 

Artiodactyle,  or  even-toed  Ungiilata,  comprise  pigs,  hippo- 
potami, camels,  sheep,  oxen,  &c.,  amongst  living  animals. 

They  are  divided  into  Ruminant  and  Non-ruminating  animals  : 
the  latter  gxoup,  equivalent  to  the  "Suina"  of  the  table  on 
page  265,  includes  the  Pigs  (Siud(i'),  Ilipixijwtamkhc,  and  Anu- 
j)lot?itrld(P. 

The  Ruminants  are  divided  into  three  groups  :  (i.)  The  Tvurfu- 
lidte  (small  deer  of  Southern  Asia),  which  foiTu  a  connecting  link 
between  the  Anoplotherium  (itself  a  link  between  the  Pigs  and  the 
true  Ruminants)  and  the  Pccura  ;  (ii.)  Pccora  (sheep,  oxen,  &c.) ; 
(iii.)  Cumdida. 

In  Artiodactyle  Ungulata  the  premolars  differ  markedly 
both  in  size  and  pattern  fi-om  the  true  molars. 

Of  those  Artiodactyle  Ungulates  which  are  not  ruminants 
the  common  pig  may  be  taken  as  an  example. 

3       13       3 
The  dental  formula  is  i_c— p  —  m— . 
3       1^3       3 

The  position  of  the  upper  incisors  is  peculiar,  the  two 
central  upper  incisors,  separated  at  their  bases,  being  inclined 
towards  one  another  so  that  their  apices  are  in  contact ;  the 
third  pair  are  widely  separated  from  the  inner  two  pairs  of 
incisors.  The  lower  incisors  are  straight,  and  are  implanted 
in  an  almost  horizontal  position  :  in  both  upper  and  lower 
jaws  the  third  or  outermost  incisors  are  much  smaller  than 
the  others. 

The  lower  incisors  are  peculiar  in  having  upon  their  upper 
surfaces  a  strongly  pronounced  sharp  longitudinal  ridge  of 
enamel,  which  gets  obliterated  by  wear. 

An  interval  separates  the  incisors  from  the  canines,  which 
latter  are  very  much  larger  in  the  male  than  in  the  female, 
and  in  the  wild  boar  than  in  the  domesticated  animal. 
Castration  arrests  the  further  development  of  the  tusks ;  the 
peculiarities  as  to  size  and  direction  which  characterise  the 


326  A    MANUAL    OF   DENTAL    ANATOMY. 

tusks  of  the  adult  animal  are  not  represented  in  the  canines 

of  the  milk  dentition,  al)out  which  there  is  not  much  that 

4 
is  noteworthy,  save  that  the  yoimg  pig  has  dec.  m  —  ,  of 

which  the  first  remains  in  place  till  the  permanent  dentition 
is  nearly  complete,  and  then  falls  out  without  having  any 
successor ;  or  it  may  perhaps  be  regarded  as  a  permanent 
tooth  which  has  had  no  predecessor. 

The  form  and  direction  of  the  canines  are  alike  peculiar  ; 


Fig.  139  ('). 


^NM.  Size 


the  upper  canine,  which  in  its  curvature  describes  more 
than  a  semicircle,  leaves  its  socket  in  a  nearly  horizontal 
direction,  with  an  incHnation  forwards  and  outwards.  After 
rounding  past  the  upper  lip  its  terminal  point  is  directed 
upwards  and  inwards.  The  enamel  upon  the  lower  surface 
of  the  tusk  is  deeply  ribbed  :  it  does  not  uniformly  cover  the 
tooth,  but  is  disposed  in  three  bands.  The  lower  canines 
are  more  slender,  of  much  greater  length,  and  by  wear 
become  more  sharply  pointed  than  the  upper  ones  :  they 
pass  in  front  of  the  latter,  and  the  worn  faces  of  the  twO' 
correspond. 

The  lower  canine  is  in  section  triangular,  one  edge  being 

(')  Upper  and  lower  teeth  of  Wikl  Boar  (Sus  scrofa).     In  tliis  specimen 
the  tusks  are  not  so  largely  developed  as  they  sometimes  may  be  seen  to  be. 


THE  TEETH  OF  ARTIODACTYLE  UXGULATA.       327 

directed  forwards,  and  its  sides  being  nearly  flat.  Enamel 
is  confined  to  the  internal  and  external  anterior  snrfaces  ; 
the  posterior  surface,  which  plays  against  the  upper  canine, 
is  devoid  of  enamel ;  the  tooth  is  kept  constantly  pointed  by 
the  obliquity  with  which  its  posterior  surface  is  worn  away. 
The  tusks  of  a  boar  are  most  formidable  weapons,  and  are 
capable  of  disembowelling  a  dog  at  a  single  stroke,  but  they 
are  greatly  exceeded  by  those  of  the  African  w^art-hog  (Phaco- 
choerus),  which  attain  to  an  immense  size. 

In  the  domestic  races  the  tusks  of  the  boars  are  much 
smaller  than  in  the  wild  animal,  and  it  is  a  curious  fact 
that,  in  domestic  races  which  have  again  become  wild  the 
tusks  of  the  boars  increase  in  size,  at  the  same  time  that 
the  bristles  become  more  strongly  pronounced.  Mr.  Darwin 
suggests  that  the  renewed  growth  of  the  teeth  may  perhaps 
be  accounted  for  on  the  principle  of  correlation  of  growth, 
external  agencies  acting  upon  the  skin  and  so  indirectly 
influencing  the  teeth. 

As  in  most  artiodactyles,  the  teeth  of  the  molar  series 
increase  in  size  from  before  backw^ards :  thus  the  first  pre- 
molar or  milk  molar  has  a  simple  wedge-shaped  crown,  and 
two  roots  ;  the  second  and  third  by  ti'ansitional  characters 
lead  to  the  fourth  premolar,  which  has  a  broad  crown  with 
two  principal  cusps,  and  has  foiu-  roots. 

The  first  true  molar  has  four  cusps  divided  from  one 
another  by  a  crucial  depression  ;  and  the  cingulum  in  front, 
and  yet  more  markedly  at  the  back,  is  elevated  into  a 
posterior  transverse  ridge.  In  the  second  molar  the  trans- 
verse ridge  is  more  strongly  developed,  and  the  four  cusps 
are  themselves  somewhat  divided  up  into  smaller  accessory 
tubercles. 

The  last  molar  measures,  from  front  to  l)ack,  nearly  twice 
as  much  as  the  second ;  and  this  great  increase  in  size  is 
referable  to  a  gi-eat  development  of  the  part  corresponding 
to  the  posterior  ridge   or  cingulum    of  the   second  molar, 


328 


A    JfANUAL    OF  DENTAL    ANATOMY. 


which  has  become  transformed  into  a  great  many  subsidiary 
tubercles. 

That  such  is  a  correct  interpretation  of  its  nature  is  indi- 
cated by  our  being  able  to  trace  the  four  principal  cusps, 
though  modified  and  not  divided  off,  in  the  front  part  of  the 
tooth,  of  which,  however,  they  do  not  constitute  more  than 
a  small  part.  Those  Ungulates  in  which  the  surfaces  of  the 
molar  teeth  are  covered  by  rounded  or  conical  cusps,  are 
termed  "  bunodonts,"  in  contradistinction  to  those  which 
present  crescentic  ridges  on  the  masticating  surface  of  their 
molars,  and  which  go  by  the  name  of  "  selenodonts." 

In  the  Wart-hog  (Phacochoerus),  the  genus  with  very  large 


Fig.  140  C). 


^cO 


canines,  the  disproportion  between  the  last  true  molar  and 
the  other  teeth  is  yet  more  striking. 

(^)  Upper  and  lower  teeth  of  Phacochoerus.  In  the  upper  jaw,  the  last 
two  premolars,  and  the  much- worn  fii-st  true  molar  remain.  In  the  lower 
all  have  been  shed  off,  save  the  last  two  true  molars.  From  a  specimen 
in  the  Museum  of  the  Eoyal  College  of  Surgeons. 


TRE  TEETH  OF  ARTIODJGTYLE  VNQVLATA.       329 

In  antero-posterior  extent  the  third  molar  equals  the  first 
and  second  true  molars  and  the  third  and  fourth  premolars 
(the  -whole  number  of  teeth  of  the  molar  series  possessed  by 
the  animal)  together. 

"Wlien  a  little  worn  its  surface  presents  about  thirty  islands 
of  dentine,  surrounded  by  rings  of  enamel,  the  interspaces 
and  the  exterior  of  the  whole  being  occupied  by  cementum. 
Of  course,  prior  to  the  commencement  of  wear,  each  of  these 
islands  was  an  enamel-coated  cusp. 

The  Wart-hog's  dentition  has,  however,  another  instructive 
peculiarity  ;  the  first  true  molar  is  in  place  early,  and  be- 
comes much  worn  down  (this  is  true,  in  a  less  degree,  of  the 
common  pig,  and  indeed^of  most  Ungulata).  Eventually  it 
is  actually  shed ;  the  same  fate  later  befalls  the  third  pre- 
molar and  second  true  molar,  so  that  the  dentition  in  an 
aged  specimen  is  reduced  to  the  fourth  premolar  and  the 
third  true  molar  alone,  and  eventually  to  the  last  true 
molai's  alone.  Thus,  both  in  the  great  complexity  of  the 
back  molars  and  the  fact  that  the  anterior  teeth  are  worn 
out  and  then  discarded,  the  Wart-hog  affords  a  parallel  to 
the  anomalous  dentition  of  the  elephant. 

As  has  already  been  noticed,  the  upper  canines  in  the 
boar  turn  outwards  and  finally  iipwards,  so  as  to  pass  out- 
side the  upper  lip ;  this  peculiarity  in  direction,  yet  more 
marked  in  Phacochoerus,  attains  its  maximum  in  the  Sus 
babirussa. 

This  creature,  strictly  confined  to  the  Malay  Archipelago, 
where  it  frequents  woody  places,  has  (in  the  male)  the  upper 
and  lower  canines  developed  to  an  enormous  extent.  The 
upper  canines  are  turned  upwards  so  abiaiptly  that  they 
pierce  the  upper  lip,  instead  of  passing  outside  it  as  in 
other  Suidce,  preserve  a  nearly  upright  direction  for  some 
little  distance,  and  then  curve  backwards,  so  that  their 
points  are  directed  almost  towards  the  eyes. 

The  lower  canines  are  less  aberrant  in  direction  and  in 


330 


A    MANUAL    OF  DENTAL    ANATOMY. 


shape,  being  somewhat  triangular  in  section,  but  they  also 
are  of  very  great  length,  and  pass  upwards,  far  above  the 
level  of  the  snout ;  their  points  are  also  directed  backwards, 

FtG.  141  {'). 


iNat.Siz, 


but  have  in  addition  an  outward  inclination.  The  canines 
are  devoid  of  enamel,  and  grow  from  persistent  pulps,  a 
fact  which  sometimes  has  a  disastrous  result,  for  the  tip 
of  the  tooth,  occasionally  taking  a  wrong  direction,  re-enters 
the  head  or  the  jaws  of  the  animal. 

Their  length  is  very  great ;  the  animal  is  smaller  than 
the  domesticated  pig,  but  its  canines  attain  a  length  of 
eight  or  ten  inches.  Their  use  is  a  matter  of  conjecture; 
the  position  of  the  upper  tusks  has  suggested  the  idea  that 
they  may  serve  as  a  protection  to  the  creature's  eyes,  as  it 
seeks  its  food,  consisting  of  fallen  fruits,  amongst  the 
brushwood.  But  were  that  the  case  the  female  also  would 
probably  have  them,  which  is  not  the  case ;  and  although 

{})  Skull  of  Sus  babirussa  (male).  The  upper  incisor.s  have  been  lost 
from  the  specimen  figured  :   they  are  much  like  those  of  a  pig. 


THE  TEETH  OF  AETIODACTYLE  UXGULATA.       331 


in  old  animals  they  are  often  broken  ott",  it  is  not  certain 
that  they  are  mucli  employed  in  fighting.  Its  other  teeth 
are  in  no  resj^ects  remarkable. 

Hippopotamus. — The  dental  characters,  as  well  as  others, 
indicate  the  athnity  of  the  Hippopotamus  to  the  SuidcB. 

.2        1        4        3 
1        e  —  p       m   — . 
■1        1^4         3 

The  incisors  are  tusk-like,  and  bear  but  little  resemblance  to 
those  of  most  other  mamuialia ;  they  are  nearly  cylindrical, 
blimtly  pointed  at  their  apices  by  the  direction  of  wear, 
which  is  in  some  measm-e  determined  by  the  partial  distri- 
bution of  the  enamel,  which  is  laid  on  in  longitudinal 
bands  in  the  upper  teeth,  but  merely  forms  a  terminal 
cap  on  the  lower  incisors. 

The  upper,  standing  widely  apart,  are  implanted  nearly 
vertically  :  the  lower  incisors,  of  which  the  median  pair  are 
exceedingly  lai'ge,  are  implanted  horizontally. 

The  canines  are  enormous  teeth ;  the  lower,  as  in  the 
Hog,  is  trihedral,  and  is  kept  pointed  in  the  same  manner ; 
the  upper  canines  are  not  so  long,  and  the  portion  exposed 
above  the  gum  is  but  short. 

The  incisors  and  canines  are  all  alike  teeth  of  persistent 
growth. 

The  premolars,  of  which  the  first  is  lost  early  (being 
perhaps  a  milk  molar  like  the  similar  tooth  in  the  pig) 
are  smaller  and  simpler  teeth  built  up  on  the  same  type 
as  the  ti'ue  molars. 

These  latter,  especially  when  worn,  have  a  very  charac- 
teristic double  trefoil  pattern ;  the  four  cusps,  in  the  first 
instance,  were  separated  by  a  deep  longitudinal  and  a  still 
deeper  transverse  groove ;  each  cusp  was,  moreover,  tri- 
lobed;  the  first  resiilt  of  wear  is  to  bring  out  the  appear- 
ance of  four  trefoils ;  next,  when  the  longitudinal  furrow  is 
worn    away,    two  four-lobed  figures  result ;    and  finally  all 


A    MANUAL    OF  DENTAL    ANATOMY. 


pattern  becomes  obliterated,  and  a  plain  field  of  dentine 
snrrounded  by  enamel  alone  remains. 

The  teeth  of  the  Hippopotamus  are  subject  to  a  great 
amount  of  attrition,  as  is  well  shown  by  a  specimen  pre- 
sented to  the  museum  of  the  Odontological  Society,  in  which 
the  molar  teeth  are  all  excessively  worn.  The  Hippopotami 
use  their  incisors  and  canine  tusks  for  the  purpose  of  up- 
rooting aquatic  plants,  of  which  their  food  mainly  consists : 
the  roots  of  these  are  of  course  mixed  up  with  much  sand, 
which  wears  down  the  teeth  with  great  rapidity.  The  larger 
incisors  and  the  canines  are,  and  for  centuries  have  been, 
articles  of  commerce,  the  ivory  being  of  very  dense  substance 
and  useful  for  the  manufacture  of  small  objects. 

Anoplotheridse  are  an  extinct  (Eocene  and  Miocene) 
family,  linking  together  the  Pigs  and  the  Pecora. 

Fig.  142  {'). 


Anoplotherium  is  a  genus  of  interest  to  the  odontologist 
because  jt  possessed  the  full  typical  mammalian  dentition, 
as  far  as  the  number  of  the  teeth  went ;  the  teeth  were 
of  nearly  uniform  height,  none  strongly  differentiated  from 
those  nearest  to  them  ;  and  they  were  set  in  close  contiguity 
with  one  another,  so  that  there  was  no  "diastema." 

The  lower  molar  teeth  of  the  anoplotherium  are  built  up 

^)  Side  view  of  the  dentition  of  Anoplotherium  (after  Owen). 


THE  TEETH  OF  ARTIODACTYLE  UNGULATA. 


333 


on  the  same  type  as  those  of  the  rhinoceros  (page  317), 
and  present  the  double  crescent ;  the  upper  molars  are  also 
referable  to  the  same  fundamental  forms,  though  the  dif- 
ference is  greater.  The  lamiute  (transverse  ridges)  oblique 
in  the  rhinoceros,  are  in  anoplotherium  still  more  oblique,  so 
that  they  become  more  nearly  parallel  with  the  outer  wall, 
and  an  accessory  pillar  is  developed  at  the  inside  of  the 
anterior  laminte. 

Not  very  widely  removed  from  the  anoplotherium  is  the 
Oreodon,  an  Ung-ulate  of  Eocene  age. 


Fig.  145  (-). 


Like  a  good  muny  tertiary  Ungulates  (both  artiodactyle 
and  perissodactyle)  it  had  the  full  typical  number  of  teeth, 
forty -four ;  but  its  interest  to  the  odontologist  is  enhanced 
by  the  co- existence  of  strongly  marked  canines  with  molars 
very  much  like  those  of  ruminants,  a  gToup  almost  always 
devoid  of  canines. 

In  the  upper  jaw  oieodon  had 

.31  4         3 

1  —  c  pm  —  m  — 


i.e.  the  typical  numljer  of  each  kind  of  teeth.  But  in  the 
lower  jaw  the  fii"st  four  teeth  are  like  incisors,  and  the 
tooth  which  is  like  a  canine  is  not  the  tooth  cori'esponding 
to  the  upper  canine,  but  to  the  small  upper  fii'st  premolar. 

C)  Upper  and  lower  teeth  of  Oreodou  Culbertsonii  after  Leidy  (Smith- 
oniau  Coutributions,  1852). 


334  A    MANUAL    OF   DENTAL    ANATOMY. 


This  is  a  fair  illustration  of  the  fact  that  although  in 
nature  it  is  generally  the  same  tooth  which  is  modified  to 
perform  the  function  of  a  canine,  it  is  not  invariably  the 
same ;  for  here  in  the  same  animal  are  two  different  teeth 
in  the  upper  and  lower  jaw  thus  respectively  modified. 

And  as  they  are  diff'erent  teeth,  it  happens  that  the  upper 
canine  closes  in  front  of  the  lower. 

There  is  reason  to  believe  that  there  was  some  difierence 
in  the  size  of  canines  between  the  male  and  female  oreodon. 

The  hollow-horned  ruminants  (sheep  and  oxen  and  ante- 
lopes), and  likewise  almost  all  the  solid  homed  ruminant 
(deer)  have  the  following  dental  formula  : — 

.00  3         .3 

'■6'   1?P   3-^3' 

The  lower  incisors  are  antagonised  not  by  teeth,  but  by 
a  dense  gum  w^hich  clothes  the  fore  part  of  the  upper  jaw ; 
if  a  sheep  is  watched  as  it  feeds,  it  will  be  seen  to  grasp  the 
blades  of  grass  between  the  lower  teeth  and  the  gum,  and 
then  to  tear  them  off"  by  an  abrupt  movement  of  the  head, 
as  it  would  be  impossible  for  it  to,  strictly  speaking,  bite 
it  ofl:: 

The  anomaly  of  the  entire  absence  of  upper  incisors  w^as 
held  to  have  been  diminished  by  the  statement  of  Goodsir, 
who  believed  that  uncalcified  tooth  germs  were  to  be  found 
in  the  fojtuses  of  many  species.  As  this  was  precisely  what 
might  have  been  expected,  it  has  since  that  time  passed 
current  as  an  established  fact ;  but  recently  M.  Pietkewickz, 
working  in  the  lalioratory  of  M.  Ch.  Robin,  has  absolutely 
denied  the  occurrence  of  even  the  earliest  rudiments  of 
tooth  germs  in  this  situation,  after  an  examination  of  a 
series  of  foetuses  of  the  sheep  and  cow,  ranging  even  from 
the  earliest  periods.  (Journal  d' Anatomic,  par  C.  H.  Robin, 
1873,  p.  452.)  Since  meeting  with  this  statement  I  have 
had  no  opportunity  of  vei'ifying  this  matter  myself. 


THE  TEETH  OF  ARTIODACTYLE  UNGULATA.       335 


Grouped  with  the  six  incisors  of  the  lower  jaw,  and  in  no 
respect  differing  from  them,  rise  the  pair  of  teeth  which  are 
very  arbitrarily  termed  "  canines."  As  I  cannot  attempt  to 
do  more  in  these  pages  than  give  the  most  bare  outline  of 
generally  well-known  facts,  I  have  retained  the  usual  dental 

0        Q 
formula,  i  -^    ^  ~Y  '  ^^^^^^  under  protest,  as  I  do  not  con- 
sider the   "  canine "  to  have  any  such  distinct  existence  as 
would  justify  our  calling  a  tooth  which  is  so  obviously  refer- 
able to  the  incisors  by  any  distinctive  name. 

Although  the  absence  of  upper  canine  teeth  is  a  very 
general  characteristic  of  ruminants,  rudimentary  canines 
exist  in  some  deer,  and  I  am  indebted  to  the  kindness  of 
Sir  Victor  Brooke,  a  high  authority  iipon  the  Cerriche,  for 
the  following  : — 

"  The  upper  canines  are  present  in  both  sexes  in  all  the 
species  of  cervidce,  with  the  exception  of  Alces,  Rangifer, 
Dama,  some  smaller  species  of  Rusa,  Axis,  Capreolus,  Caria- 
cus,  Blastocerus,  Coassus,  and  Pudu.  The  upper  canines, 
when  present,  are  with  the  notable  exception  of  Moschus, 
Elaphodus,  Cervulus,  and  Hydropotes,  small  laterally  com- 
pressed rudimentary  teeth.  Their  crowns  are  in  about  the 
same  stage  of  reduction  as  the  crowns  of  horses'  canines, 
but  their  roots  are  relatively  much  more  reduced."  Hence 
they  are  often  lost  in  dried  skidls,  and  it  has  generally  been 
supposed  that  but  few  deer  possessed  canines  at  all. 

The  hornless  musk  deer  possesses  iipper  canines  of  most 
formidable  dimensions,  while  the  female  has  very  small 
subcylindrical  canines. 

The  male  pigmy  musk  deer  (Tragulus)  has  large  canines 
of  persistent  growth,  the  female  small  canines  with  closed 
roots. 

The  Indian  Muntjac  deer  (Cervulus)  has  somewhat  small 
horns,  which  are  perched  upon  persistent  bony  pedicles, 
and  it  has  upper  canines  which  are  curved  outwards  from 


336  A    MANUAL    OF   DENTAL    ANATOMY. 

beneath  the  upper  lip,  much  as  are  the  tusks  of  a  boar ; 
they  do  not,  however,  grow  from  persistent  pulps,  and  are 
absent  in  the  female. 

Cuvier  first  pointed  out  that  there  was  a  relation  between 
the  presence  of  horns  and  the  absence  of  canine  teeth  ;  the 
latter,  serving  as  weapons   for  sexual  combat  solely,   and 


Fig.  144  (i). 


being,  probably,  in  no  other  way  of  service  to  the  animal, 
are  not  required  by  an  animal  provided  with  powerful  antlers 
or  horns,  whereas  the  absolutely  hornless  musk  deer  would 
be  totally  unprovided  with  weapons  of  offence  w^ere  it  not  for 
his  canines.  To  the  musk  deer  and  the  muntjac  must  be 
added  Swinhoe's  water  deer,  Hydropotes  inermis,  and  Michie's 
deer,  Elaphodus  cephalopus,  another  small  hornless  species, 
of  which  the  males  are  furnished  with  formidable  canine 
teeth. 

Although,  with  the  foregoing  exceptions,  all  the  deer, 
oxen,  sheep,  antelopes,  and  the  giraffe,  animals  constituting 
the  greater  number  of  the  "  Ruminautia,"  are  without  canine 
teeth,  yet  in  the  remaining  family,  the  Camelidce,  tusk-like 
canines  are  met  with. 

It  is  a  character  of  the  Artiodactyle  Uugulata  that  the 

(')  Cranium  of  male  Musk  Deer  (Mosclius  moschiferus). 


THE  TEETH  OF  ART  WD  ACT  YLE  UNGULATA.       337 


premolar  teeth  are  of  decidedly  simpler  form  than  the 
molars ;  indeed  in  the  ruminants  the  premolars  may  be 
said  each  to  correspond  to  one  half  of  a  true  molar. 

The  dentition  of  the  ordinary  ruminant  having  been 
more  or  less  illustrated  by  the  example  of  the  musk  deer 
(minus  its  great  canines),  the  Camel  may  be  selected  as 
illustrating  the  peculiarities  of  the  molar  series. 

The  Camel  is  possessed  of  an  upper  incisor,  and,  as  has 
already  been  noticed,  of  canines. 


113         3 

c        n   —  m   — . 
3        1^2         3 


The  first  two  pairs  of  upper  incisors  are  absent,  but  the 


Fig.  145  (i). 


imt.si=, 


third  or  outermost  pair  ai'e  preseiit,  and  are  rather  caniniform 
in  shape.  In  quite  young  skulls  six  upper  incisors  are  present, 
but  the  two  inner  pairs  are  lost  very  early.  The  canines  are 
strong  pointed  teeth,  and  the  lower  canine  stands  well  apart 
from  the  three  incisors  of  the  lower  jaw,  unlike  the  fourth 
tooth  in  front  of  the  mandible  of  typical  pecora  (see  Fig. 
115). 

(')  Upper  and  lower  teeth  of  a  Camel, 


338  A    MANUAL    OF   DENTAL    ANATOMY. 


The  first  premolars  are  absent  altogether ;  the  second 
premolars,  following  the  canines  after  an  interval,  ax*e 
pointed  caniniform  teeth.  The  third  premolar  is  some- 
times lost  early,  but  the  fourth  persists. 

The  molars  of  the  Camel  are  of  the  "  Selenodont  "  type  ; 
their  derivation  from  forms  already  alluded  to  will  be 
sufficiently  obvious  to  the  reader  who  has  mastered  the 
descriptions,  and  their  double  crescentic  crowns,  may 
be  taken  as  fair  examples  of  simple  ruminant  patterns, 
accessory  pillars,  etc.,  being  added  in  some  of  the  other 
groups. 

In  all  true  Ruminants  the  last  true  molar  of  the  lower 
jaw  has  a  third  lobe  ('),  and  the  line  of  the  outer  surface  of 
the  row  of  teeth  is  rendered  irregular  by  the  anterior  edge 
of  each  tooth  projecting  outwards  slightly  more  than  the 
posterior  border  of  the  one  in  front  of  it.  And  the  devia- 
tions in  the  patterns  of  the  surfaces  of  the  molar  teeth  are 
so  constant  and  so  characteristic  that,  although  the  common 
ruminant  pattern  is  preserved  in  all,  it  is  often  possible  to 
refer  an  individual  tooth  to  its  right  genus. 

The  Paiminants  all  have  a  well-developed  milk  dentition, 
which  serves  the  animal  for  a  long  time,  indeed  iintil  after 
it  has  attained  to  its  adult  dimensions ;  thus  a  sheep  has  not 
completed  the  changing  of  its  teeth  till  the  fifth  year,  and 
a  calf  till  the  fourth  year.  But  the  first  permanent  molar 
is  in  them,  as  in  so  many  other  animals,  the  first  of  the 
permanent  set  to  be  cut,  and  comes  up  in  its  place  at  the 
sixth  month  (in  the  lamb),  and  hence  has  a  long  period  of 
wear  before  any  of  the  other  second  teeth  are  cut.  Conse- 
quently the  first  permanent  molar  is,  as  is  seen  in  Fig.  145, 
invariably  worn  down  to  a  much  greater  extent  than  the 
other  permanent  teeth  ;  in  the  specimen  figured  it  has  been 

(')  Sir  Victor  Brooke  informs  nie  that  Neotragus  liemprichii,  a  small 
Abyssinian  antelope,  has  only  two  lobes  to  the  third  lower  molar. 


TOXODONTIA.  339 


worn  down  below  the  inflections  of  enamel,  so  that  it  has 
lost  its  roughened  grinding  sm-face,  and  is  reduced  to  a 
smooth  area  of  dentine. 

Not  much  is  known  of  the  structure  of  the  dentul  tissues  of 
the  Ungulata  which  calls  for  mention  in  an  elementary  work. 
The  thick  cement  of  the  crown  of  the  teeth  of  the  Horse, 
and  indeed  of  most  of  the  group  which  possess  thick  cement, 
contains  many  "  encapsuled  lacunee,"  and  is  developed  from 
a  distinct  cement  organ  of  cartilaginous  consistence  (see 
page  144). 


TOXODOXTIA. 


The  existing  ungulate  animals  form  only  a  small  propor- 
tion of  those  once  peopling  the  earth,  and  many  extinct  forms 
have  been  discovered,  which  while  having  afl&nities  with  the 
Ungulata,  can  yet  hardly  be  classified  under  any  existing 
order.  For  example,  Toxodon,  a  creature  equalling  the 
Hippopotamus  in  size,  which  was  discovered  by  !Mr.  Darwin 
in  late  tertiaiy  deposits  of  Soiith  America,  has  a  dentition 
recalling  in  some  respects  the  Bruta,  in  others  the  Rodents. 

It  possessed  in  the  upper  jaw  two  pairs  of  incisors,  the 
median  pair  small,  the  outer  exceedingly  large,  with  per- 
sistent pulps,  and  long  curved  sockets  extending  back  to  the 
region  of  the  molars,  just  as  in  existing  Rodents. 

In  the  lower  jaw  there  were  three  pairs  of  incisors,  sub- 
equal  in  size,  and  gi'owing  from  persistent  pulps ;  they 
resemble  the  incisors  of  Rodents  in  haviug  a  partial  invest- 
ment with  enamel,  but  differ  from  them  in  being  prismatic 
in  section,  and  in  having  the  enamel  disposed  on  two  sides 
of  the  prism. 

The  molai's  were  also  very  remarkable ;  they  grew  from 
persistent  pulps,  and  had  curved  sockets,  but  the  cmwature 
of  these  was  in  the  reverse  dnection  to  that  which  obtains 

z  2 


340  A    MANUAL    01'    DENTAL    ANATOMY. 

in  Rodents,  i.e.,  the  convexity  was  outwards,  and  the  apices 
of  their  roots  almost  met  in  the  middle  line  of  the  palate  ; 
it  was  this  peculiarity  that  suggested  the  name. 

Another  peculiarity  in  the  molar  teeth,  in  which  they 
stand  quite  alone,  is  that,  like  incisors,  they  have  a  partial 
investment  with  enamel ;  those  refeiTed  to  the  premolar 
series  having  it  confined  to  their  outer  surfaces,  while  the 
three  back  teeth  of  the  molar  series  had  a  plate  also  laid  on 
to  their  inner  surfaces  ;  there  were  seven  molar  teeth  above, 
and  six  below. 

In  the  interval  between  the  incisor  and  molar  series 
canines  have  been  found  in  the  lower  jaw  ;  they  were  sharp 
edged,  and  had  a  pai'tial  distribution  of  enamel  over  their 
surface.  In  an  upper  jaw  alveoli  for  canines  were  found,  but 
the  teeth  themselves  are  not  known. 


DINOCERATA. 


In  the  same  region  which  yielded  the  toothed  birds 
(Eocene  formations  of  Wyoming),  the  remains  of  many  huge 
animals  have  been  discovered,  for  which  new  orders  have  been 
proposed  by  Prof.  Marsh  ("  American  Journal  of  Science  and 
Art,"  1876),  it  being  impossible  to  classify  them  under  any 
existing  order.  The  Dinocerata  were  creatures  nearly  as 
large  as  Elephants,  and  presenting  some  general  resemblance 
to  them  in  general  form ;  they  were  remarkable  for  the 
relative  smallness  of  their  brains,  which  coidd  aj^pai-ently 
have  been  drawn  through  the  canal  of  the  vertebral  column. 
They  present  points  of  resemblance  to  the  Perissodactyle 
Ungulata,  and  also  to  the  Proboscidea,  to  which  they  were 
at  first  referred,  though  their  affinities  are  rather  with  the 
former. 

The  dental  formula  was 

.01  33 

x_c-prm-m-^. 


DINOCERATA. 


341 


In  Prof.  Marsh's  words  "  The  superior  caniues  are  loug, 
decurved,  trenchant  tusks.  They  are  covered  with  enamel, 
and  their  fangs  extend  upwards  into  the  base  of  the  maxil- 
lary horn-core.  There  is  some  evidence  that  these  tusks 
were  smaller  in  the  females.  Behind  the  canines  there  is  a 
moderate  diastema.  The  molar  teeth  are  very  small.  The 
crowns  of  the  superior  molars  are  formed  of  two  transverse 
crests,  separated  externally,  and  meeting  at  their  inner 
extremity.  .  The  first  true  molar  is  smaller  in  this  specimen 
than  the  two  preceding  premolars.  The  last  upper  molar  is 
mvich  the  largest  of  the  series. 

"  The  lower  jaw  in  Dinoceras  is  as  remarkable  as  the  skull. 
Fig.  116  ('). 


Its  most  peculiar  features  are  the  posterior  direction  of  the 
Q)  Upper  and  lower  jaws  of  Dinoceras  (Marsh). 


A    MANUAL    OF   DENTAL    ANATOMY. 


condyles,  hitherto  unknown  in  Ungulata,  and  a  massive 
decxirved  process  on  each  ramus  extending  downward  and 
outward  below  the  diastema. 

"The  position  of  the  condyles  was  necessitated  by  the  long 
upper  tusks,  as,  with  the  ordinary  ungulate  articidation,  the 
mouth  could  not  have  been  fully  opened.  The  low  position 
of  the  condyle,  but  little  above  the  line  of  the  teeth,  is  also 
a  noteworth}^  character.  The  long  pendant  processes  were 
apparently  to  protect  the  tusks,  which  otherwise  would  be 
very  liable  to  be  bix»ken.  Indications  of  similar  processes 
are  seen  in  Smilodon  and  other  Carnivores  Avith  long  upper 
canines.  With  the  exception  of  these  processes  the  lower 
jaw  of  Dinoceras  is  small  and  slender.  The  symphysis  i& 
completely  ossified.  The  six  incisors  were  contiguous,  and 
all  directed  well  forward.  Just  behind  these,  and  not 
separated  from  them,  were  the  small  canines,  which  had  a 
similar  direction.  The  crowns  of  the  large  molars  have 
transverse  crests,  and  the  last  of  the  series  is  the  largest." 

It  would  appear  possible  that  the  eminences  shown  in  the 
figure,  and  spoken  of  as  "  maxillary  horn-cores,"  may  be 
merely  the  extended  sockets  of  the  teeth,  which  w^ould  other- 
wise have  had  an  implantation  inadequate  to  their  length  ■. 
they  are,  however,  described  as  solid,  except  at  their  bases, 
where  they  are  perforated  for  the  fang  of  the  canine  tusk, 
which  wotdd  look  as  though  they  were  truly  horn-cores ; 
moreover  the  Brontotheridse  had  horn-cores  equally  peculiar 
in  position  {i.e.,  on  the  maxillary  bones). 

Yet  another  new  oi'der,  Tillodoxtia,  comprising  several 
genera,  has  been  proposed  by  Prof.  Marsh  for  the  Wyoming 
fossil  remains,  to  receive  forms  which,  though  not  amongst  the 
biggest,  are  "  amongst  the  most  remai'kable  yet  discovered  in 
American  strata,  and  seem  to  combine  characters  of  several 
distinct  groups;  viz.,  Carnivora,  Ungulata,  and  Rodentia, 
In  Tillotherium,  Marsh,  the  type  of  the  order,  the  skull  has 
the  same  general  form  as  in  the  Bear,  but  in  its  structure 


DINOCERATA. 


343 


resembles  that  of  the  Ungulata.  Its  molar  teeth  are  of  the 
ungulate  type,  the  canines  are  small,  and  in  each  jaw  there 
is  a  pair  of  large  scalpriform  incisors,  faced  with  enamel  and 
gi-owing  from  persistent  pulps,  as  in  the  Rodents.  The 
second  pair  of  incisors  are  small,  and  have  not  persistent 
pulps.     The  adult  dentition  is  as  follows — 


1  3        3 

-pmi— m 


"  There   are   two   distinct  families,  Tillotheridce  (perhaps 
identical  with  Ancldppodontidce),  in  which  the  large   incisors 

Fig.  147  (')• 


grew  from  persistent  pulps,  while  the  molars  had  roots  ;  and 
the  Styli7iodontid(e,  in  which  all  the  teeth  have  persistent 
pulps. 

One  genus  (Dryptodon),  known  only  by  the  lower  jaw,  had 
six  teeth,  described  as  "  clearly  incisors,"  the  two  inner  pairs 
of  which  are  small  and  cylindrical,  the  outer  of  enormous 
size,  faced  in  front  only  with  enamel,  and  with  persistent 
pulps  carried  back  under  the  premolars. 

(')  Upper  and  lower  jaws  of  Tillotherium  (Marsli). 


CHAPTER   XI. 

THE  TEETH  OF  SIRENIA,  HYRACOIDEA,  PROBOSCIDEA,  AND 
RODENTIA. 

THE    TEETH    OF    SIRENIA. 

More  nearly  connected  with  the  Unguhita  than  with  any 
other  order,  but  still  rather  widely  removed  from  them, 
stands  the  limited  order  of  Sirenia,  aquatic  mammals 
formerly  termed  Herbivorous  Cetacea,  a  term  rather  ob- 
jectionable, as  they  are  not  very  nearly  allied  to  the  true 
Cetacea. 

The  order  is  now  represented  by  two  genera  only,  the 
Dugongs  (Halicore)  and  the  Manatees  (Manatus),  but  a 
third  gemis  (Rhytina)  has  only  become  extinct  within  about 
a  century.  Their  teeth,  and  other  points  in  their  organiza- 
tion indicate  that  they  are  more  nearly  allied  to  the  Ungu- 
lata  than  to  any  other  group,  though  their  peculiarities  are 
such  as  to  elevate  them  to  the  rank  of  a  distinct  order. 
They  are  of  large  size,  and  frequent  shallow  water,  such 
as  the  mouths  of  great  rivers,  their  food  consisting  of  sea- 
weed and  aquatic  plants. 

The  dentition  of  the  Dugong  is  in  several  respects  a  very 
interesting  one  :  the  front  part  of  the  upjjer  jaw,  consisting 
in  the  main  of  the  intermaxillary  bones,  bends  abruptly 
downwards,  forming  an  angle  with  the  rest  of  the  jaw. 
This  deflected  end  of  the  jaw  carries  two  tusks,  of  each  of 
which  the  greater  part  is  buried  within  the  alveolus.  The 
tusk  has  an  investment  of  enamel  over  its  front  and  sides, 
but  on  the  posterior  surface  of  cementum  only,  so  that  in 


THE    TEETH    OF   SIRENIA. 


345 


the  disposition  of  the  three  structures  it  recalls  the  charac- 
teristics of  a  Rodent  incisor,  like  which  it  is  worn  away 
obliquely  so  as  to  keep  a  constantly  sharp  edge,  and  like 
which  it  gi-ows  from  a  persistent  pulp. 

In  the  female,  the  tusks  (incisors)  do  not  project  from  the 


Fig.  148  ('). 


gum,  their  pulp  cavities  are  closed,  and  the  investment  of 
enamel  is  complete  over  the  crown  of  the  tooth. 

The  sloping  surface  of  the  upper  jaw  is  opposed  b}-  the 
region  of  the  symphysis  of  the  lower  jaw,  which  is  of  un- 
usual depth.  In  this  deflected  part  of  the  lower  jaw  there 
are  eight,  or  ten  (four  or  five  on  each  side)  shallow  and 
rather  irregularly-shaped  sockets,  in  which  curved  distorted 


(')  Side  view  of  cranium  and  lower  jaw  of  a  Dugong  (Halicore  Indicus). 
From  a  specimen  in  the  ^Museum  of  the  Royal  College  of  Surgeons.  The 
surface  of  the  deflected  portion  of  the  lower  jaw,  with  its  sockets  for 
rudimentary  teeth,  shown  both  in  front  and  in  profile  view,  is  indicated  hy 
the  letter  a  ;  the  con-esponding  surface  of  the  upper  jaw  by  the  letter  h. 


346  A    MANUAL    OF   DENTAL    ANATOMY. 

teeth  may  be  fouud  in  a  fresh  specimen,  but  it  must  not 
be  from  too  aged  an  animal,  as  they  become  eventually  eaten 
away  by  a  process  of  absorption. 

These  abortive  teeth  are  excellent  examples  of  rudi- 
mentary teeth,  as  not  only  are  they  stunted,  and  even 
ultimately  removed  by  absorption,  but  they  are  actually 
covered  in  by  a  dense  horny  plate  which  clothes  this  part 
of  the  jaw,  and  so  are  absolutely  functionless  ('). 

These  horny  plates,  in  their  structure  analogous  to  whale- 
bone, are  possessed  also  by  the  Manatee  and  Rhytina ;  on 
the  free  surface  they  are  beset  with  stiff  bristles,  and  are 
throughout  built  up  of  hair-like  bodies  welded  together  by 
epithelium. 

Behind  the  region  covered  in  by  the  horny  plates,  the 
Dugong  has  five  molar  teeth  on  each  side,  of  simple  form, 
like  those  of  the  Edentata,  and  consisting  of  dentine  and 
cementum  only. 

By  the  time  the  last  molar  is  ready  to  come  into  place, 
the  first  of  the  series  is  being  removed  by  absorption  of  its 
root  and  of  its  socket.  In  aged  specimens  only  two  molars 
remain  on  each  side  of  the  jaws. 

The  Dugong  is  also  peculiar  as  having  a  single  deciduous 
tooth  :  namely,  a  predecessor  to  the  incisive  tusks  ;  but  it 
has  been  doubted  whether  it  be  not  rather  a  rudimentary 
incisor  than  a  milk  tooth. 

The  molar  teeth  of  the  Manatee  are  much  more  nume- 
rous and  more  complex  in  form,  and  they  approach  to  the 
configuration  of  the  teeth  of  the  Tapir  very  closely. 

The  Manatee  has  as  many  as  forty-four  molars,  which 
are  not,  however,  all  in  place  at  one  time,  the  anterior  ones 
being  shed  l)cfore   the   posterior  are   come    into    place ;  no 

(1)  Similar  rudimentary  teeth  are  found  in  the  corresponding  deflected 
part  of  the  jaw  of  the  young  Manatee,  to  the  number  of  twelve.  (Gervais, 
"Histoire  Nat.  des  Mammiferes,"  vol.  ii.,  p.  312.) 


THE    TEETH    OF   SIEENIA. 


vertical  succession  is  known  to  occur  among  them.  There 
are  no  incisors  nor  canines,  but  there  are  horny  plates  in 
the  front  of  the  mouth  like  those  of  the  Dugong. 

The  extinct  Rhytina,  formerly  abundant  about  Behring's 
Straits,  was  altogether  without  teeth. 

It  has  been  mentioned  that  the  teeth  of  the  Manatee  are 
tapiroid  in  external  form ;  they  also  possess  peculiarities  in 
minute  structure,  which  are  unusual  in  mammalian  teeth,  but 
which  are  common  to  them  and  to  the  Tapirs.  In  examining 
some  teeth,  I  found  that  the  dentine,  to  all  intents  and  pur- 
poses, of  the  hard  unvascular  variety,  was  pemieated  by  a 
system  of  larger,  or  "  vascular "  canals,  which  were  ar- 
ranged with  miich  regularity,  and  passed  out  from  the  pulp 
cavity  to  the  periphery  of  the  dentine,  where  they  commu- 
nicated with  one  another.  The  dentinal  tubes  did  not 
radiate  from  these  vascular  canals  ;  they,  so  to  speak,  take 
no  notice  of  them,  so  that  there  is  an  ordinaiy  unvascular 
dentine  with  a  system  of  capillary- conveying  channels  as 
well.  It  is  interesting  to  find  that  the  primd  facie  external 
resemblance  of  the  teeth  of  the  Tapir  is  fully  borne  out  by 
minute  histological  structure,  and  it  certainly  suggests  that 
the  resemblance  is  not  accidental,  but  has  some  deeper 
significance. 

The  enamel  of  the  Manatee  is  also  somewhat  remarkable 
for  the  absolute  straightness  of  its  enamel  prisms  in  many 
parts  of  the  tooth. 

The  molar  teeth  of  the  Dugong  consist  of  a  central  axis 
of  vaso-dentine,  a  much  lai'ger  mass  of  ordinary  unvascular 
dentine,  and  a  thick  layer  of  cementum,  but  they  do  not 
share  the  peculiarities  of  the  jManatee's  tooth. 


348 


A    MANUAL    OF   DENTAL    ANATOMY. 


THE    TEETH    OF    HYRACOIDEA. 

The  Biblical  coney  (Hyrax),  an  animal  as  large  as  a 
rabbit,  must  not  be  passed  over  without  mention,  as  its 
dentition  has  been  indirectly  the  source  of  much  contro- 

Fid.  149  (1). 


versy.  So  far  as  the  pattern  of  its  molar  teeth  goes,  it 
corresponds  closely  with  Rhinoceros,  and  was  hence  classed 
in  close  proximity  to  that  genus  by  Cuvier.  But  a  more 
extended  sui'vey  of  its  characters  has  led  to  its  being  placed 
in  a  separate  order;  it  is  a  good  example  of  the  danger 
which  attends  relying  upon  any  single  character,  such  as 
the  pattern  of  the  teeth,  as  being  alone  a  sufficient  basis  for 
classification. 

Q)  Skull  of  the  Hyrax. 


THE    TEETH    OF   PROBOSCIDEA.  349 

All  observers,  however,  are  not  agreed  as  to  its  position  ; 
it  certainly  presents  affinities  -with  Perissodactyla,  and  also 
with  the  Rodents,  also,  perhaps,  with  the  Insectivora,     The 

dental  formula  is    i  —  c  —  prm  —  m  — . 

Seen  from  the  side,  the  dentition  bears  some  resemblance 
to  that  of  a  Rodent,  because  the  large  size  of  its  central 
incisors,  which  grow  fi-om  persistent  pulps,  are  chisel-edged, 
and  are  fui-nished  with  a  very  thick  coat  of  enamel  on  their 
anterior  faces  :  the  second  pair  of  incisors,  which  are  small, 
are  soon  lost.  But  Hyi-ax  has  the  full  typical  number,  pre- 
molars and  molars,  and  the  patterns  of  the  teeth  are  closely 
similar  to  those  of  the  Rhinoceros. 

In  the  lower  jaw  the  middle  incisors  are  small,  and  the 
outer  ones  largely  developed,  and  all  persist :  their  crowns 
are  in  a  manner  trilobed,  and  they  pass  in  ordinary  closure 
of  the  mouth  behind  the  upper  incisors,  where  they  are  met 
by  a  dense  pad  of  gum. 

THE    TEETH    OF    PROBOSCIDEA. 

At  the  present  day  the  Elephant  stands  alone,  removed 
by  many  striking  peculiarities  from  the  Ungulata,  to  which 
it  is  more  nearly  allied  than  to  other  orders  ;  but  in  former 
days  the  order  Proboscidea  was  represented  by  a  good  many 
genera,  was  widely  distributed  over  the  globe,  and  tran- 
sitional forms  linking,  the  elephant  with  somewhat  less 
aberrant  mammalia  were  not  wanting.  In  this  group  the 
incisors  grow  from  persistent  pulps,  and  form  conspicuous 

tusks  ;  the  Elephant  has    i  — ,  the  Mastodon  has  i  _,   the 
^  0  1 

Dinotherium  i  — 
1 

Two  striking  features  characterise  the  dentition  of  the 


A    MANUAL    OF   DENTAL    ANATOMY. 


Elephant ;  the  enormous  length  of  the  incisor  tusks,  and 
the  peculiar  displacement  from  behind  forwards  of  the  molar 
teeth,  by  which  it  results  that  not  more  than  one  whole 
molar,  or  portions  of  two,  are  in  place  at  any  one  time. 

The  upper  tusks  are  preceded  by  small  deciduous  teeth ; 
this  is  well  established,  though  it  has  been  recently  denied  by 
Sanderson  ("Wild  Beasts  of  India"  ).  When  first  cut  they 
are  tipped  with  enamel,  but  the  enamel  cap  is  soon  worn  off, 
and  the  remainder  of  the  tusk  consists  of  that  modification 
of  dentine  known  as  "  ivory,"  and  of  a  thin  external  layer  of 
cement. 

In  the  Indian  elephant  the  tusks  are  not  so  large  as  in  the 
African  species  :  and  the  tusks  of  the  female  are  rery  much 
shorter  than  those  of  the  male.  In  the  African  elephant,  no 
such  difference  in  size  has  been  established ;  and  amongst  In- 
dian elephants  males  are  sometimes  met  with  which  have  tusks 
no  larger  than  the  females  of  corresponding  size ;  they  go 
by  the  name  of  "  Mucknas."  This  peculiarity  is  not  always 
transmitted,  and  it  is  known  that  in  Ceylon  tuskless  sires 
sometimes  beget  "  tuskers."  Amongst  the  Ceylon  elephants 
the  possession  of  large  tusks  by  the  male  is  an  exceptional 
thing,  Sanderson  stating  that  only  one  in  three  hundred  has 
them,  while  amongst  51  Indian  elephants  only  five  were 
tuskless.  The  tusks  are  formidable  weapons,  and  great 
dread  of  a  "  tusker,"  is  shown  by  elephants  less  well  armed. 

A  male  makes  use  of  his  tusks  for  all  soi-ts  of  piu-poses  ; 
thus  when  a  tamed  one  is  given  a  rope  to  pull,  he  will,  by 
way  of  getting  a  good  purchase  upon  it,  pass  it  over  one 
tusk  and  grasp  it  between  his  molar  teeth. 

The  largest  tusks  were  possessed  by  the  Mammoth,  the 
remains  of  which  are  so  abundant  in  Siberia  ;  these,  which 
were  strongly  curved,  and  fonned  a  considerable  segment 
of  a  circle  with  an  outward  inclination,  so  as  to  well  clear  the 
sides  of  the  head,  attained  the  lengih  of  13  feet,  and  a  weight 
of  200  lbs.  each. 


THE    TEETH  OF  PROBOSCIDEA. 


A  pair  of  African  tusks  exhibited  at  the  Great  Exliibition 
of  1851  weighed  325  lbs.,  and  measured  eight  feet  sis  inches 
in  length,  and  22  inches  in  circumference,  but  the  average 
tusks  imported  from  Africa  do  not  exceed  from  20  lbs.  to 
50  lbs.  weight.  Indian  elephants  seldom  have  tusks  attain- 
ing very  large  dimensions  ;  one  was,  however,  shot  by  Sir 
Victor  Brooke  with  a  tusk  8  feet  long,  weighing  90  lbs. 

The  surfaces  of  the  tusks  of  the  female  are  often  deeply 
excavated  about  the  level  of  the  edge  of  the  gum,  and  are 
sometimes  so  weakened  from  this  cause  that  they  break  off. 
My  friend  Mr.  Moseley  tells  me  that  he  was  informed  by  the 
late  Major  Rossall,  who  as  a  sportsman  had  great  knowledge 
of  Indian  elephants,  that  the  tusks  of  all  the  females  he  has 
ever  seen  are  so  affected,  and  that  the  larva)  or  pupse  of  a 
dipterous  insect  are  often  found  bedded  in  the  gum,  and 
attached  to  the  surface  of  the  tusk.  There  is  a  specimen  of 
a  female  elephant's  tusk  with  the  pupre  attached  in  the 
Museum  of  the  Ro3-al  College  of  Surgeons.  It  would  be  a 
matter  of  interest  to  ascertain  whether  the  larva  really  eats 
away  the  tusk,  or  whether  the  wasting  of  the  tusk  be  due 
to  absorption  set  up  by  the  irritated  gum. 

The  tusks  of  the  elephant  are  implanted  in  long  and 
stout  sockets,  and  grow  from  persistent  pulps  throughout 
the  lifetime  of  the  animal. 

In  the  Indian  elephant  about  one  half  of  the  length  of  the 
tusk  is  implanted,  and  in  young  animals  the  pulp  cavity 
extends  beyond  the  implanted  portion,  but  in  older  animals 
it  does  not  extend  nearly  so  far.  A  knowledge  of  its  extent 
is  necessary,  seeing  that  the  tusks  of  captive  elephants  have 
to  be  shortened  from  time  to  time ;  this  operation  is  by 
some  done  frequently,  by  others  only  at  long  intervals,  such 
as  ten  years,  in  which  case  a  large  and  valuable  segment  of 
ivory  is  cut  off,  and  the  end  of  the  tusk  bound  with  metal 
to  prevent  it  from  splitting. 

Tusks  sometimes  exemplify  on  a  large  scale  the  results  of 


J    MANUAL    OF   DENTAL    ANA  TOM V. 


injury  to  the  growing  pulj),  as  it  is  of  no  unfrequent  oc- 
currence that  elephants  which  have  been  shot  at  and 
wounded  escape. 

The  thin  walls  of  the  tusk  near  to  its  open  end  do  not 
offer  very  much  resistance  to  the  entrance  of  a  bullet ;  the 
result  of  such  an  injury  is  not,  as  might  have  been  expected, 
the  death  of  the  pulp,  but  in  some  cases  abscess  cavities 
become  formed  in  the  neighbourhood  of  the  injury,  while 
in  others  less  disturbance  is  set  up,  the  bullet  becomes 
enclosed  in  a  thin  shell  of  secondary  dentine,  or  sometimes 
lies  loose  in  an  irregular  cavity,  and  round  this  the  normal 
*'  ivory  "  is  deposited ;  upon  the  outside  of  the  tusk  no  indi- 
cation of  anything  unusual  is  to  be  seen,  so  that  the  bullets 
thus  enclosed  are  found  by  ivory  turners  only  when  sawing 
up  the  tusk  for  use. 

As  the  tusk  grows,  that  which  was  once  in  the  pulp 
cavity,  and  within  the  alveolus,  comes  to  be  at  a  distance 
from  the  head,  and  in  the  midst  of  solid  ivory. 

As  an  example  of  the  extent  of  injury  from  which  a 
tooth  pulp  is  capable  of  recovery,  may  be  cited  a  specimen 
now  deposited  in  the  museum  of  the  Odontological  Society, 
by  Mr.  Bennett,  to  whom  I  am  indebted  for  permission  to 
figure  it. 

It  is  to  be  presumed  that  a  trap  was  sot  with  a  heavily 
loaded  spear,  or  that  it  was  dropped  by  a  native  from  a 
tree,  with  the  intention  of  its  entering  the  brain  of  the 
elephant  as  it  was  going  to  water,  both  of  these  methods  of 
killing  elephants  being  practised  in  Africa.  But  in  this  case 
the  spear  penetrated  the  open  base  of  the  growing  tusk, 
which  looks  almost  vertically  upwards  (see  fig.  150),  and 
then  the  iron  point  appears  to  have  broken  off".  This  did 
not  destroy  the  pulp,  but  the  tooth  continued  to  grow,  and 
the  iron  point,  measuring  no  less  than  7i  by  1-J-  inches, 
became  so  completely  enclosed  that  there  was  nothing  upon 
the  exterior  of  the  tusk  to  indicate  its  presence. 


THE    TEETH    OF   PROBOSCIDEA. 


353 


I  am  told  by  Mr.  Erxleben  that  he  is  acquainted  with 
another  instance  in  which  a  spear  head  had  become  com- 
pletely enveloped  in  ivory. 


Fig.  150('). 


There  is  also  a  specimen  of  a  javelin  head  solidly  im- 
,bedded  in  ivory  in  the  Museum  of  the  Royal  College  of 
Surgeons. 

Ivory  is  one  of  the  most  perfectly  elastic  substances 
known,  and  it  is  on  this  account  that  it  is  used  for  billiard 
balls ;  it  owes  its  elasticity  to  the  very  small  size  of  the 
dentinal  tubes  and  the  frequent  bends  (secondary  curva- 
tures) which  they  make ;  to  the  aiTangement  of  the  tubes 
the  peculiar  engine-turning  pattern  of  ivory  is  due.  It 
diflfers  from  other  dentine  in  its  containing  from  40  to  43 
per  cent,  of  organic  matter  (human  dentine  contains  only 
about  25),  and  in  the  abundant  concentric  rows  of  inter- 
globular spaces.  Along  these  ivory  -when  it  decomposes 
breaks  up,  so  that  a  disintegrated  segment  of  a  tusk  con- 
sists of  detached  concentric  rings ;  in  this  condition  many 
mammoth  teeth  are  found,  although  sometimes  where  they 
have  remained  frozen  and  protected  from  the  air  until  the 
time  of  their  discovery  they  are  hardly  affected  by  the  lapse 


{})  Iron  spear-head,  irremovably  fixed  in  the  interior  of  a  tusk,  believed 
to  be  from  an  African  Ele^jhant.  From  a  specimen  in  the  possession  of 
Mr.  Bennett. 


354  A    MANUAL    OF   DENTAL    ANATOMY. 


of  the  thousands  of  years  which  haA'e  gone  by  since  their 
possessors  perished. 

The  trade  in  ivory  is  quite  an  important  one,  the  Boaixl 
of  Trade  returns  for  1879,  giving  9,414  cwts.,  of  the  value  of 
^406,927,  as  the  quantity  brought  to  this  coiuitry. 

The  best  ivory  is  that  which  comes  from  equatorial 
Africa ;  Indian  ivory  is  not  so  highly  esteemed,  and  Mam- 
moth ivory  is  so  uncertain  in  its  degree  of  preservation 
that  it  does  not  find  a  ready  sale,  even  though  some  samples 
almost  attain  the  quality  of  recent  ivory. 

The  last  remains  of  the  pulp  are  converted  into  dentine 
in  which  a  few  vascular  canals  persist ;  this  of  ct)urse  occu- 
pies the  centre  of  the  task,  and  is  small  in  amount. 

Six  molar  teeth  are  developed  on  each  side  of  the  jaw  by 
the  elephant,   and,  arguing  from  analogy,  they  are  some- 

3  3 

times     classified     thus — milk    molars   -    true     molars    - ; 

3  3 

occasionally  a  rudimentary  tooth  in  front  brings  up  the 
number  to  seven  on  each  side.  But  the  peculiarity  of  their 
mode  of  succession  renders  such  a  classification  merely 
arbitraiy,  so  far  as  the  elephant  itself  is  concerned,  and  it 
depends  upon  analogy  with  the  teeth  of  the  mastodon. 
Though  the  elephant  has,  during  the  course  of  its  life, 
twenty-four  molars,  they  are  not  all  in  place,  nor  indeed 
are  they  all  actually  in  existence  at  the  same  time.  Only 
one  whole  tooth  on  each  side,  or  portions  of  two  (when  the 
front  one  of  the  two  is  nearly  worn  out),  are  in  use  at  the 
same  time.  After  a  tooth  has  been  in  use  for  some  time, 
and  is  worn  down,  a  new  tooth  comes  up  to  take  its  place 
from  behind  it,  and  absori;)tion  in  the  old  tooth  being  set  up, 
it  is  shed  off,  and  the  new  tooth  pushes  forward  into  its 
place  (see  fig.  151).  Each  successive  tooth  is  of  greater 
size  than  its  predecessor ;  thus  in  the  Indian  elephant  the 
first  tooth  having,  on  an  average,  four  transverse  plates ; 
the  second  eight,  the  third  twelve,  the  fourth  twelve,  the 


THE    TEETH    OF   PROBOSCIDEA. 


355 


fifth  sixteen,  the  sixth  from  tweuty-four  to  twenty-seven- 
In  the  African  elephant,  in  which  the  individual  plates  are 
much  broader,  they  arc  fewer  in  number  (sec  page  361). 


Fig.  151  ('). 


10  \at  Size 


A  reference  to  the  accompanying  figure  will  indicate  how 
the  succession  takes  place.  The  tooth  in  reserve  occupies  a 
position  at  an  angle  to  that  in  use ;  as  it  moves  forwards 


(')  Side  view  of  skull  of  young  Indian  Elephant.  The  teeth  in  use  are 
the  second  and  third  of  the  molars  whicli  displace  one  another  from  be- 
hind forwards  ;  the  anterior  of  these,  corresponding  to  a  milk  molar  in 
other  animals,  is  nearly  worn  out  ;  the  residiial  fragment  is  separately  re- 
presented on  the  left.  The  tusk,  of  which  only  a  short  piece  can  be 
shown,  is  indicated  within  the  socket  by  dotted  lines,  by  which  also  the 
form  of  the  pulp  cavity  is  majiped  out. 

A  A  2 


356  A    MANUAL    OF   DENTAL   ANATOMY. 

and  (in  the  iipper  jaw)  downwards  its  track  forms  almost 
the  segment  of  a  circle.  Thus  its  anterior  corner  is  the 
first  to  come  into  use,  at  a  time  when  the  position  of  the 
"whole  tooth  is  still  exceedingly  oblique,  and  the  greater  part 
of  it  is  still  within  the  socket. 

The  teeth  as  first  formed  consist  of  detached  plates  of 
identine  coated  with  enamel,  the  tops  of  which  are  mammil- 
lated  ;  these  only  coalesce  after  a  considerable  portion  of 
their  depth  has  been  formed,  and  that  portion  of  the  tooth 
has  lieen  reached  in  which  there  is  a  common  pulp  cavity ; 
here  dentine  is  continuous  from  end  to  end  of  the  tooth. 

Just  as  the  cusps  of  a  human  molar  are  separate  when 
first  calcified,  so  these  exaggerated  cusps  or  plates  of  an 
elephant's  tooth  are  separate  from  one  another  till  a  great 
part  of  their  length  is  completed,  and  they  only  coalesce 
when  they  reach  the  level  of  the  common  pulp  chamber ;  in 
point  of  fact  the  elephant's  tooth  is  mainly  made  up  of  its 
cusps,  the  remaining  portion  being  insignificant. 

Several  of  these  detached  plates,  such  as  the  one  here 
figured,  are  to  be  found  at  the  back  of  the  largest  teeth 
even  at  a  time  when  the  front  corner  has  been  erupted  and 
has  come  into  wear. 

That  the  tooth  is  thus  being  built  up  only  as  it  is  required 
is  of  obvious  advantage  to  the  animal  in  diminishing  the 
weight  to  be  carried,  and  is  also  an  economy  of  space. 

The  teeth  when  they  begin  to  be  erupted  do  not  at  once 
come  into  use  over  their  whole  surface,  but  they  come  for- 
ward in  an  oblique  position  so  that  the  front  of  the  tooth 
has  been  in  use  for  some  time,  and  its  plates  have  been  con- 
siderably worn  down,  before  the  back  of  the  tooth  has 
become  exposed  at  all.  Nay  more,  in  the  case  of  the  larger 
molars  the  front  of  the  tooth  is  actually  in  use  at  a  time 
when  its  back  is  not  yet  completed. 

In  the  elephant  there  is  no  vertical  succession  of  teeth 
■whatever;  the  manner  of  succession  usual  amongst  mammals 


THE    TEETH    OF   PROBOSCIDEA. 


357 


has  in  them  given  place  to  a  succession  from  behind,  the 
older  teeth  being  pushed  out  forwards.  Had  the  elephant 
always  been  as  isolated  a  form  as  it  now  appears  to  be,  it 
would  have  been  very  uncertain  how  its  six  molars  shoidd 


be  classified.  But  it  happens  that  proboscideans  formerly 
existed  in  which  this  pecular  succession  from  behind  was  to 
be  found,  at  the  same  time  that  the  ordinary  vertical  suc- 
cession was  not  quite  lost,  and  amongst  these  creatures  (the 
mastodons)  we  are  able  to  say  with  certainty  which  of  the 
teeth  are  milk  molars,  which  are  pre-molars,  and  which  are 
true  molars.  And  as  the  mastodons  pass  by  insensible  gra- 
dations into  the  elephants,  so  that  the  line  of  demarcation 
between  the  tv/o  genera  is  an  arbitrary  one,  we  can  tell 
which  of  the  mastodon's  teeth  correspond  to  each  one  of  the 
six  molars  of  the  elephant. 

Mastodon. — In  the  later  tertiary  .periods  this  genus,  ap- 
proximating in  its  dental  and  other  characters  to  the  true 


(')  Isolated  plate  (=  exaggerated  cusp)  of  an  Elephant's  tooth,  prior  to 
its  coalescence  with  neighbouring  jjlates  ;  at  the  top  are  seen  its  terminal 
mammillated  processes,  one  of  which  has  been  ciit  ofi'  to  show  the  central 
.irea  of  dentine,  surrounded  by  enamel  ;  at  the  base  would  be  the  open 
I)ulp  cavity,  not  shown  in  the  figure. 


358  A    MANUAL    OF   DENTAL    ANATOMY. 

elephant,  was  widely  distributed  over  the  world.    The  dental 

formula  is  not  quite  the  same  for  all  the  genus,  for  in  some 

no  premolars  existed. 

10  2  3         3 

i.  _  c.  -  prm.  -  milk  molars  -  m.  - 

1       0^        2  3         3 

The  upper  incisors  formed  nearly  straight  tusks,  seven  or 
eight  feet  in  length  ;  the  lower  incisors  also  grew  out  hori- 
zontally from  the  front  of  the  jaw,  but  in  some  species  the 
lower  tusks  are  rudimentary,  are  lost  early,  or  are  altogether 
absent,  thus  more  nearly  approaching  to  the  condition  met 
with  in  the  elephant. 

The  several  molar  teeth  of  the  Mastodon  increased  in 
size  from  before  backwards.  The  crowns  were  built  up  of 
deep  and  strongly  pronounced  transverse  ridges,  of  which 
the  last  molar  had  the  largest  number.  The  apices  of  the 
ridges,  before  being  at  all  worn,  were  divided  up  into  several 
blunt  nipple-like  (mastoid)  processes,  upon  which  the 
enamel  was  thick  and  dense,  but  the  cement  was  thin,  so 
that  the  interspaces  of  the  processes  were  not  filled  up  level 
by  the  latter  tissue,  as  in  the  elephant. 

Very  definite  roots  were  formed  to  the  molars,  the  wear- 
ing down  of  the  teeth  being  met  by  the  worn  teeth  being 
shed  oflP  altogetlier  from  the  front  of  the  series,  whilst  new 
teeth  were  added  to  the  back.  Thus,  just  as  in  the  elephant, 
the  whole  number  of  teeth  were  not  in  place  at  one  time. 
jSTot  more  than  three  were  in  use  at  one  time,  and  by  the 
time  the  last  and  largest  molar  was  cut,  there  was  but  one 
tooth  remaining  in  front  of  it,  and  even  this  Avas  soon  lost, 
the  dentition  thus  being  reduced  to  a  single  molar  on 
each  side. 

As  the  succession  of  the  molars  in  the  Mastodon  affords 
a  clue  to  the  nature  of  the  grinders  of  the  elephant,  it  is 
necessary  to  add  a  few  words  about  it.  Some  Mastodons 
had  three  milk  molars,  of  which  the  last  two  were  vertically 
displaced  by  premolars,  just  as  in  most  other  mammals, 


THE    TEETH   OF  PROBOSCIDEA.  359 

but  the  first  milk  molar  was  not  so  replaced  (Mastodon 
angustidens).  There  ajjpear  to  have  been  Mastodons  in 
which  no  vertical  succession  at  all  took  place,  i.e.,  in  which 
there  were  no  j^remolars,  and  others  in  which  there  was 
but  one. 

No  doubt  can  be  entertained  as  to  the  homologies  of  the 
teeth,  even  in  those  Mastodons  which  are  not  known  to 
have  any  vertical  succession,  because  analogy  with  those 
other  species  in  which  the  second  and  third  molai's,  counted 
from  the  front,  were  vertically  displaced  by  nearly  func- 
tionless  premolars,  tells  us  that  the  three  front  molars  ai"e 
milk  molars.  Now  elephants  develop  six  molar  teeth  on 
each  side  ;  the  elephant  is  in  the  same  case,  quoad  its  molars, 
as  the  Mastodon  Ohioticus,  which  had  do  vertical  succes- 
sion, so  that  we  thus  know  the  elephant's  grinders  to  be 

,       3        3 

dm.  -  m.  - 

3        3 

Dr,  Falconer  mentions  an  elephant  from  the  Sewalik  Hills 
(E.  planifrons)  in  which  two  rudimentary  pre-molars,  of  no 
functional  importance,  actually  existed,  and  so  the  deter- 
mination of  the  elephant's  working  teeth  as 

,       3        3 

dm.  _  m.  - 

3        3 

rests  not  only  upon  analogy,  but  upon  actual  observation. 

The  Dinotherimu,  a  large  animal,  not  unlike  the  Sirenia 
in  the  character  of  its  cranium,  which  was  probably  of 
aqiiatic  habits,  was  remarkable  for  possessing  large  tusks,  by 
analogy  known  to  be  incisors,  in  its  lower  jaw,  none  being 
present  in  the  upper  jaw.  The  tusks  projected  do waiwards 
at  right  angles  with  the  body  of  the  jaw,  and  were  curved 
backwards.  The  portion  of  jaw  about  the  symphysis  was 
<leflected  downwards,  so  as  to  aftbrd  an  adequate  implanta.- 
tion  for  these  anomalous  tusks. 


360 


A    MANUAL    OF   DENTAL    ANATOMY. 


The  Dinotherium  was  as  largo  as  an  elephant,  and  the 

downward  pointmg  tusks  were  about  2  feet  in  length  ;   as, 

however,  tusks  of  only  half  this  length  were  found  in  somo 

jaws  of  identical  dimensions  and  in  other  respects  similar, 

it  is  believed  that  the  male  Dinotherium  had  larger  tusks 

than  the  female.     The  molar  teeth,  much  like  those  of  a 

tapii-,  need  not  detain  us. 

.002 

1.  -  c.  -p.  -  m.  - 

The  succession  was  vei'tical,  as  in  other  mammals,  and  it 

had  dm.  - 
o 

But  the  Dinotherium,  Mastodon,  and  Elephant,  present 
us  with  a  very  instructive  series  of  modifications  in  which 
we  see  how  the  excessively  complex  grinder  of  the  Indian 
elephant  was  attained  to  by  degrees. 

The  molar  of  the  Dinotherium  resembles  that  of  a  tapii- 
somewhat;  it  has  not  any  very  great  exaggeration  of  its 
cusps,  and  does  not  deviate  very  widely  from  the  form  of 
many  other  mammalian  teeth. 

The  tooth  of  Mastodon  has  its  cusps  or  ridges  more 
numerous  and  more  pronounced,  as  is  seen  in  the  accom- 
panying figiu-e. 

Fig.  153  (>). 

h        c        R  c 


Other  Mastodons  have  more  numerous  ridges  upon  the 
teeth,  and  the  African  elephant  has  as  many  as  ten  upon 

(')  Molar  tootli  of  Mastodon. 


THE    TEETH    OF   PROBOSCIDEA. 


S61 


i-\is. 


Fir,.  156  (^'). 


(')  Molar  of  African  Elephant.  E.  Enamel.  D.  Dentine.   C.  Cementuni. 

(-)  Molar  tooth  of  African  Elephant,  showing  the  form  of  its  roots,  &c. 
n.  Dentine,     c.  Cementum.     e.  Enamel. 

(•*)  Molar  tooth  of  an  Asiatic  Elephant,  showing  the  transverse  plates  of 
dentine  bordered  by  enamel. 


362  A    MANUAL    OF   DENTAL    ANATOMY. 


its  last  01-  larger  molar,  although  in  it  the  ridges  are  in- 
dividually wide  and  strongly  pronounced. 

In  the  Indian  elephant  the  ridges  or  plates  are  still  more 
numerous,  the  roots  very  inconspicuous  and  the  whole 
formed  into  a  solid  block  by  cementura. 

The  gradual  increase  in  complexity  in  the  "  ridge  formula" 
(or  number  of  ridges  in  each  tooth),  of  the  molars,  is  well 
seen  in  the  following  table,  from  Prof.  Flower's  Hunterian 
lecture  ("Nature,"  March  2,  1876);  it  is  a  corrected  table 
taken  from  Dr,  Falconer's  "  Pahcontological  Memoirs." 

Milk  Molars.  TnieMolars.  Total. 
I.    II.  III.    I.    II.  III. 


Dinotherium  giganteum  . 

.     1 

2     3 

3     2     2 

13 

Mastodon  (Trilophodoii)  americanus    . 

.     1 

2     3 

3     3     4 

16 

,,         (Tetralophodon)  arvemensis 

.     2 

3     4 

4     4     5 

22 

(Pentalophodon)  sivalensis  . 

.     8 

4     .5 

.5     .0     6 

28 

Elephas  (Stegodon)  insignis      . 

.     2 

5     7 

7     8  10 

39 

,,        (Loxodon)  africanus 

.    :i 

fi     7 

7     8  10 

41 

,,                „           raeridionalis 

.     3 

(i     8 

8     9  12 

46 

,,        (Euelephas)  antiquus 

.     3 

0  10 

10  12  16 

57 

.,                .,              primigenius 

.     i 

S  12 

12  16  24 

76 

„               ,.              indicus 

.     4 

«  12 

12  16  24 

76 

Some  variability  exists  in  the  number  of  ridges,  especially 
when  they  are  very  numerous,  but  the  above  may  be  taken 
as  averages ;  and  some  species  intermediate  in  the  "  ridge 
formula "  have  been  since  discovered,  thus  M.  pentelici  and 
M.  andium  bridge  the  distinction  between  Trilophodon  and 
Tetralophodon,  and  Elephas  melitensis  comes  between 
Loxodon  and  Euelephas  (Flower). 

It  remains  to  describe,  somewhat  more  in  detail,  the 
structure  of  an  elephant's  tooth,  and  this  has  been  deferred 
till  the  last,  because  it  can  be  the  more  easily  understood 
when  the  manner  of  its  origin  has  been  mastered.  In  the 
Mastodon  the  molar  consists  of  a  crown  with  strong  cusps, 
standing  apart,  and  with  marked  roots ;  in  the  African 
elephant  that  part  which  consists  of  cusps  has  become  the 


THE    TEETH   OF  PROBOSCIDEA. 


greater  bulk  of  the  tooth,  the  roots  are  comparatively  in- 
significaut,  and  the  interspaces  of  the  cusps  are  filled  up 
with  cementum.  The  molar  of  the  Indiau  elephant  con- 
sists of  a  larger  number  of  yet  more  elongated  and  flattened 
cusps,  so  that  the  greater  i)art  of  the  tooth  is  made  up  of 
these  flattened  plates,  fused  together  with  cementum,  and  so 
forming  a  strong  and  solid  mass ;  the  roots  are  compara- 
tively inconspicuous. 

When  the  tooth  is  a  little  worn  each  plate  consists  of  an 
area  of  dentine  sm-rounded  by  enamel.  The  interspaces 
of  the  series  of  plates  are  wholly  filled  up  by  cementum ; 
the  summits  of  each  plate  Avere  originally  mammillated, 
and  divided  up  into  moi'c  numerous  blunt  processes  than 
the  corresponding  pai-ts  of  the  tooth  of  a  Mastodon ;  when 
the  tooth  comes  into  use  tlie  rounded  tips  are  soon  worn 
ofi',  and  the  grinding  surface  of  the  tooth  then  consists  of 
narrow  transverse  bands  of  dentine,  suiTounded  by  enamel, 
and  of  cementum  in  their  interspaces.  The  difference  in 
hardness  between  these  three  tissues  preserves  a  constant 
rough  surface,  owing  to  their  unequal  rate  of  wear.  In 
their  wild  condition  elej^hants  eat  trees  with  succulent  juicy 
stems,  and  oftentimes  grass  torn  up  by  the  roots,  from 
which  they  roiighly  shake  out  the  adherent  earth.  In  con- 
finement, the  food  containing  less  that  is  gritty,  the  teeth 
become  polished  by  working  against  one  another,  but  the 
rate  of  wear  is  insufficient  to  keep  their  surfaces  rough  ;  for 
the  softer  cementum  does  not  get  worn  down  in  the  inter- 
spaces of  the  plates  of  dentine  and  enamel,  but  remains  on 
a  level  with  them. 

Great  though  the  size  of  the  Proboscideans  be,  they  have 
some  points  of  affinity  with  the  Eodents  in  the  great 
development  of  the  incisors,  the  vacant  interval  between 
these  and  the  molar  teeth,  and,  as  was  pointed  out  by  the 
late  Professor  Rolleston,  the  enamel  of  the  elephant's  molar 
having,   in   its    inner   portions,  a  pattern  produced  hy  the 


364  A    MANUAL    OF   DENTAL    ANATOMY. 


decussation  of  the  prisms  which  is  very  similar  to  that  de- 
scribed by  my  father  as  characteristic  of  all  the  Rodents 
save  the  Leporidse  (Hares)  and  Hystricid^o  (Porcupines). 


THE    TEETH    OF    RODENTIA. 


The  animals  belonging  to  this  order,  which  is  sharply 
defined,  are  scattered  almost  all  over  the  world ;  the  island 
of  ]\Iadagascar  is,  however,  remarkable  for  being  almost 
without  indigenous  Rodents,  as  is  the  case  also  with  Aus 
trulia,  two  facts  which  are  of  no  small  interest  to  the 
student  of  odontology. 

For  in  each  of  these  areas,  out  of  the  creatures  which  are 
there  (in  the  one  Lemurs,  in  the  other  Marsupials),  there 
has  ai'isen  a  form  so  modified  as  to  mimic  the  dentition  of 
the  true  Rodents,  viz.,  the  Cheiromys  in  Madagascar,  and 
the  Wombat  in  Australia. 

The  species  of  Rodents  are  exceedingly  numerous,  and 
the  great  majority  of  them  are  of  small  size ;  the  aquatic 
Capybara  is  far  the  largest  of  recent  Rodents. 

In  general  features  the  dentitions  of  the  numerous 
species  comprising  this  order  are  very  uniform  ;  the  incisors, 
(save  in  the  halves  and  rabbits,  in  which  there  is  an  ac- 
cessory small  pair  immediately  behind  the  large  ones)  are 
reduced  to  four  in  number,  are  of  very  large  size,  and  grow 
from  persistent  pulps.  The  jaws  for  some  little  distance 
behind  the  incisors  are  devoid  of  teeth,  while  beyond  the 
interval  the  back  teeth,  generally  not  more  than  four  in 
number,  are  arranged  in  lines  which  diverge  slightly  as  they 
pass  backward.  The  large  scalpriform,  or  chisel-like  incisors, 
extend  far  back  into  the  jaws,  and  are  much  curved,  the 
upper  incisors,  in  the  words  of  Professor  Owen,  forming  a 
larger  segment  of  a  smaller  circle  than  the  lower,  which  are 
less  curved.     The  length  and  curvature  of  these   incisors 


THE    TEETH    OF  RODENT  I  A.  365 

relieve  from  direct  pressiire  their  growing  jDulps,  which 
come  to  be  situated  far  back  in  the  jaw,  the  open  end  of  the 
lower  incisor,  for  example,  being  in  many  species  actually 
behind  the  last  of  the  molar  teeth.  The  nerve  going  to 
supply  the  persistent  pulps  is  of  very  large  size,  and,  owing 
to  the  open  end  of  tlie  tooth  having  formerly  occupied  a 
more  anterior  position  in  the  jaw,  runs  forward  beneath  the 

Fi3   157  ('). 


tooth,  and  then  bends  abruptly  backwards  to  reach  the 
tooth-pulp.  In  many  Rodents  the  enamel  of  the  front  of 
the  large  incisors  is  stained  of  a  deep  orange  colour ;  this 
colour  is  situated  in  the  substance  of  the  enamel  itself. 

The  scalpriform  incisors  terminate  by  cutting  edges,  the 
sharpness  of  which  is  constantly  maintained  b}"  the  peculiar 
disposition  of  the  tissues  of  the  tooth. 

The  investment  of  enamel,  instead  of  being  continued 
round  the  whole  circumference  of  the  tooth,  is  confined  to 
its  anterior  and  lateral  surfaces,  on  the  former  of  which  it  is 
thickest. 

It  is  said  by  Hilgendorff  (Berlin  Akad.  d.  Wiss.  Monats- 
bericht,  1865),  that  the  incisors  of  Hares  differ  from  those  of 

')  Side  view  of  skull  of  a  Rodent,  giving  a  general  idea  of  tlie  denti- 
tion of  the  order. 


366  A    MANUAL    OF   DENTAL    ANATOMY. 

all  other  Rodents  in  having  enamel  all  round  them,  although 
it  is  veiy  thin  at  the  back.  I  have  not  been  able  to  satisfy 
myself  that  the  thin  clear  layer  at  the  back  of  the  tooth  is 
enamel,  and  am  disposed  to  regard  it  as  cementum,  the  more 
so  as  it  seems  to  be  continued  a  little  way  upon  the  enamel, 
and  in  very  young  teeth  the  enamel  organ  is  confined  to  the 
anterior  surface. 

When  a  rodent  incisor  has  been  exposed  to  -weai-,  the 
anterior  layer  of  enamel  is  left  projecting  beyond  the  level 
of  the  dentine,  and  this  an-angement  results  in  a  very  sharp 
edge  being  constantly  maintained.  The  dentine  also  is 
harder  near  to  the  front  of  the  tooth  than  towards  the  back 
of  the  tooth. 

A  thin  external  coat  of  cement  is  found  upon  the  back  of 
the  tooth,  but  is  not  continued  far  over  the  face  of  the 
enamel.  In  the  marsupial  wombat  this  layer  of  cement  is 
contiimed  over  the  whole  anterior  surface  of  the  scalpriform 
incisors. 

The  molar   teeth    are    not    very    numerous;    the    mouse 
3 
family   have    usually   -  ;    the    porcupines   have    constantly 

4  G 

-,  and  the  hares  -  ;  the  Australian  water-rat  CHx- 
4  5 

dromys)  is   altogether  exceptional   in   having  so   few  as  ^. 

Observation  has  established  that  the  last  three  of  these 
teeth  are  always  true  molars,  and  that  when  there  are  more 
than  three,  the  rest  are  premolars,  and  have  had  deciduous 
predecessors. 

But  the  extent  to  which  the  milk  teeth  are  developed 
varies  much.  Mr,  Waterhouse  (Nat.  Hist,  of  Mammalia — 
Rodents,  p.  4),  has  found  the  milk  molar  still  in  place  in 
the  skull  of  a  half-grown  beaver,  while  in  the  hares  they 
are  shed  about  the  eighteenth  day  after  birth,  and  in  the 
guinea-pig  disappear  before  birth.     Deciduous  incisors  have 


THE    TEETH    OF  EODENTIA.  367 

not  been  found  in  any  of  the  group,  save  in  the  hares  and 
rabbits. 

In  the  hares  and  rabbits  there  are  four  incisors  in  the 
upper  jaw,  a  small  and  apparently  functionless  pair  being 
placed  close  behind  the  large  rodent  incisors ;  but  in  A'ery 
young  specimens  there  are  six  incisors,  of  which  the  one  pair 
are  soon  lost. 

Prof.  Huxley  (Nature,  vol.  23,  p.  228)  has  recently  written 
that  "  the  deciduous  molars  and  the  posterior  deciduous 
upper  incisors  of  the  rabbit  have  been  long  known.  But  I 
have  recently  found  that  unborn  rabbits  possess,  in  addi- 
tion, two  anterior  upper  and  two  lower  deciduous  incisora. 
Both  are  simple  conical  teeth,  the  sacs  of  which  are  merely 
embedded  in  the  gum.  The  upper  is  not  more  than  one- 
hundredth  of  an  inch  long,  the  lower  rather  lai-gcr.  It 
would  be  interesting  to  examine  foetal  guinea-pigs  in  rela- 
tion to  this  point ;  at  present  they  are  known  to  possess 
only  the  hindmost  deciduous  molars,  so  far  agi'eeing  with 
the  Marsupials." 

Hares  and  rabbits  have  six  milk  molars  in  the  upper  and 
four  in  the  lower  jaw,  which  come  into  use,  but  differ  from 
their  successors  in  forming  definite  roots  and  not  growing 
from  persistent  pulps. 

Other  rodents,  such  as  the  rat,  which  has  only  three 
teeth  of  the  molar  series  on  each  side,  and  the  Australian 
water-rat  (Ilydromys)  have  no  milk  teeth,  and  are  hence 
truly  Monophyodont. 

Moz-e  diversity  exists  in  the  premolar  and  molar  teeth  ; 
in  rodents  of  mixed  diet,  such  as  the  common  rat,  the  back 
teeth  are  coated  over  the  crown  with  enamel,  which  nowhere 
forms  deep  folds,  and  have  distinct  I'oots,  i.e.,  are  not  of 
persistent  growth ;  the  molars  of  the  rat  have  some  sort  of 
i-esemblance  to  minute  human  molars.  In  aged  specimens 
the  enamel  is  consequently  worn  off  the  grinding  surface  of 


368  A    MANUAL    OF   DENTAL    ANATOMY. 


the  crown,  -which  comes  to  be  an  area  of  dentine,  surrounded 
by  a  ring  of  enamel. 

But  in  those  whose  food  is  of  a  more  refractory  nature, 
the  molars,  like  the  incisors,  grow  from  persistent  pulps  (as 
is  exemplified  in  the  Capybara  here  figured),  and  their 
working  surfaces  are  kept  constantly  rough  by  the  enamel 
dipping  in  deeply  from  the  side  of  the  tooth,  as  may  also  be 
seen  in  the  common  water-rat.  The  inflection  of  enamel 
may  be  so  deep  as  to  divide  the  areas  of  dentine  completely 
np,  the  result  being  a  tooth  like  that  of  the  Capybara,  which 

Fig.  158  ('). 


is  composed  of  a  series  of  plates  of  dentine,  or  '  denticles,' 
surrounded  by  layers  of  enamel,  and  all  fused  together  by 
the  ccmentum.  The  result  of  this  disposition  of  the  struc- 
tures is  that  the  working  surface  is  made  up  of  enamel, 
dentine,  and  cementum,  three  tissues  of  diftercnt  hardness, 
which  will  consequently  wear  down  at  different  rates,  and 
so  maintain  its  roughness.  Various  intermediate  forms  of 
the  molar  teeth  are  met  with  ;  thus  there  are  some  in 
which  complexity  of  the  surface  is  maintained  by  folds  of 
enamel  dipping  in  for  a  little  distance,  but  which  never- 
theless after  a  time  form  roots  and  cease  to  grow.  When 
the  molar  teeth  grow  from  persistent  pulps,  they  are  always 
curved,  like  the  incisors,  with  the  eff'ect  of  relieving  the 
pulps  from  direct  pressure  during  mastication ;  and  the  last 

(')  Molar  of  Capybara,  showing  the  transverse  plates  of  dentine  and 
'  united  to  one  another  by  cementum. 


THE    TEETH   OF  llOBENTIA. 


remains  of  the  jiulps  are  converted  into  secondary  or  osteo- 
dentine,  which  thus  forms  the  central  axis  of  the  incisors,  or 
molars,  as  the  case  may  be.  In  this  tissue  vascular  tracts 
sometimes  exist,  but  it  is  altoofcther  small   in  amount,  the 


(^)- 


formation  of  true  dentine  going  on  till  the  pulp  at  that 
jiarticular  point  is  almost  obliterated. 

As  has  already  been  mentioned,  when  the  molar  series 
consists  of  more  than  three  teeth,  those  anterior  to  the 
three  true  molars  are  jDrcmolars,  which  have  disjilaced  milk 
teeth ;  but  they  do  not  differ  materially  in  size  or  form  from 
the  true  molars. 

The  form  of  the  condyle  and  of  the  glenoid  cavity  in 
Rodents  are  characteristic ;  they  arc  much  elongated  in  an 
autero-posterior    direction,  so  tliat  the  range   of  backward 

(')  Condyle  and  glenoid  cavity  of  the  Capybara,  showing  their  longitu- 
dinal direction. 


A    MANUAL    OF  DENTAL    ANATOMY. 


and  forward  motion,  made  use  of  in  gnawing,  is  very  con- 
siderable. The  Lepariihjd  are  exceptional  in  having  more 
lateral  play  than  most  Rodents.  And  the  power  of  the 
teeth  is  mai-vcllons ;  rats  will  sometimes  gnaw  holes  in 
water-pipes,  or  in  gas-pipes,  in  which  they  have  heard  water 
bubbling. 

The  general   character   of  a  llodcnt's   dentition  may  be 
illustrated  by  a  description  of  that  of  the  Capylnira. 


■iKat.^izc^ 


The  incisor  teeth  are  squarish.  They  are  wider  than 
they  are  deep,  and  are  slightly  grooved  on  their  anterior 
.surface. 

There  are  four  grinding  teeth  on  each  side,  of  which  the 
first  three  are  small,  and  with  few  cross  plates  of  dentine 
and  enamel,  but  the  fourth  is  a  very  complex  tooth,  with 
twelve  or  more  such  plates,  which  are  fused  into  a  solid 
mass  by  cementum. 

The  tooth  being  one  of  persistent  growth,  there  is  no 
common  pulp  cavity,  but  each  jilate  has  its  own. 

It  has  already  been  mentioned  (page  160)  that  the  den- 

(')  Cranium  of  CapyLara. 


THE    TEETH    OF   IIUDEXTIA.  371 

tiual  tubes  at  that  part  of  the  liodent't>  incisor  which  has 
•come  into  use  are  much  smaller  than  those  near  to  its 
growing  base,  thereby  proving  that  they  have  undergone  a 
diminution  in  calibre  at  a  time  subsequent  to  their  original 
formation.  Near  to  the  surflxce  actually  in  wear  they 
become  cut  off  from  the  pulp  cavity  by  the  conversion  of 
what  remains  of  the  pulp  into  a  laminated  granular  mass, 
so  that  the  dentine  exposed  on  the  surface  of  a  Rodent's 
tooth  must  be  devoid  of  sensitiveness,  and  the  contents  of 
the  dentinal  tubes  must  have  presumably  undergone  some 
change.  But  what  the  nature  of  the  change  in  the  contents 
of  dentinal  tubes  which  have  ceased  to  be  in  continuity  with 
a  vascular  living  pulp  may  be,  there  are,  so  far  as  I  know, 
no  observations  to  indicate. 

As  Avas  shown  by  my  fother  (Phil.  Trans.  1850),  the 
enamel  of  Rodents  is  peculiar,  and  some  little  diversity  in 
the  arrangement  of  the  prisms  exists  in  different  families  of 
the  order^  their  character  being  in  many  cases  so  marked, 
that  it  is  often  possible  to  con-ectly  refer  a  tooth  to  a  par- 
ticular family  of  Rodents  after  simple  inspection  of  its 
enamel. 

In  general  terms  it  may  be  said  that  the  enamel  is  divided 
into  two  portions,  an  outer  and  an  inner  portion  (this  is 
true  of  all  save  the  hares  and  rabbits),  and  that  the  enamel 
prisms  pursue  different  com-ses  in  these  two  portions. 

Thus  in  the  enamel  of  the  beaver,  in  the  inner  half,  nearest 
to  the  dentine,  the  prisms  of  contiguous  layers  cross  each 
other  at  right  angles,  whereas  in  the  outer  portion  they  are 
all  parallel  with  one  another. 

In  the  genera  Soiurus,  Pteromys,  Tamias,  and  Spermo- 
philus  the  enamel  fibres,  as  seen  in  longitudinal  section, 
start  from  the  dentine  at  right  angles  to  its  surface ;  in 
Castor  they  incline  upwards  at  an  angle  of  G0°,  but  preserve 
the  distinction  between  the  outer  and  inner  layers  very 
distinctlv. 


372 


A    MANUAL    OF   DENTAL    ANATOMY. 


In  tlie  MuriclcB  the  decussation  of  the  layers  in  the  inner 
part,  and  their  parallelism  in  the  outer  part  of  the  enamel 


Frr,.  \a\  (1). 


are  also  found,  but  in  addition  to  this  the  borders  of  the 
individual  prisms  are  slightly  serrated,  the  serrations  of 
contiguous  fibres  interlocking. 

In  the  porcupine  suborder  the  fibres  of  the  inner  portion 
of  the  enamel  pm-sue  a  serpentine  course,  nevertheless 
showing  indications  of  a  division  into  layers ;  they  become 
parallel  in  the  outer  portions  as  in  other  Rodents.  Small 
interspaces  are  found  amongst  the  enamel  fibres  of  the 
Porcupines. 

In  the  hares  {Leporidcn)  the  lamelliform  arrangement,  and 
the  division  into  outer  and  inner  layers,  alike  disappear. 

The  peculiarities  in  the  disposition  of  the  enamel  fibres, 
which  are  so  marked  in  the  incisors,  do  not  generally  exist 
in  the  molars  of  the  same  species. 

Many  minor  differences  in  the  arrangement  of  the  enamel 
prisms  exist,  for  a  description  of  which  I  must  refer  the 
reader  to  the  original  paper,  but  in  general  terms  it  may  be 
said  that  the  "  enamel  lamelke  have  a  different  and  distinc- 


(')  Transverse  section  of  an  incisor  of  a  Eeaver  (Castor  fiber).  The 
enamel  jirisms  of  suiierimposed  layers  cross  each  other  at  right  angles  in 
the  inner  portion  of  the  euaniel,  but  all  becoize  parallel  in  the  outer. 


THE    TEETH    OF   RODENT  I  A.  373 

tive  character  in  each  of  the  larger  groups,  and  that  the 
variety  of  structure  is  constant  throughout  the  membci's  of 
the  same  grouj) ;  we  may  take,  for  example,  the  Sciuriche, 
the  Muridoe,  and  the  Hystricidce,  in  each  of  which  the 
structure  of  the  enamel  is  different ;  and  in  each  is  highly 
distinctive."  And  further,  that  the  varieties  in  the  struc- 
ture of  the  dental  tissue,  so  far  as  they  are  known,  with  a 
few  isolated  exceptions,  justify  and  accord  with  the  classifi- 
cation of  the  members  of  the  order  given  by  Mr.  Waterhouse 
in  his  Natural  Historv  of  the  ]\Iammalia. 


CHAPTER    XII. 

THE    TEETH    OF    CARXIVORA. 

The  auiinals  grouped  together  under  the  name  of 
Carnivora  are  divided  into  two  sections,  the  Aquatic  and 
the  Terrestrial  Carnivora. 

The  teiTestrial  Carnivora  were  fomierly  classed  as  ••  digitigrade  " 
and  "  plantigi-ade,"  a  classification  exceedingly  inconvenient,  as  ifc 
left  the  greater  number  of  the  animals  to  be  classified  in  the  de- 
bateable  ground  between  the  two  extreme  types.  As  a  linear 
classification  is  impossible,  tliey  are  now  grouped  around  three 
centres  :  the  Q^luroidea,  or  cat-like  ;  the  Cynoidea,  or  dog-like  ; 
and  the  Ai'ctoidea,  or  bear-like  Carnivora  ;  and,  instead  of  taking 
the  Felidaj,  or  Cats,  as  the  tyi^e  of  the  group,  it  is  generally  con- 
sidered that  the  Dog  tribe  are  the  most  generalised  foi-m,  and  that 
the  Cats  are  an  extreme  modification  in  one  dnection,  the  Bears  in 
another. 

The   Cynoidea  comprise   the    Dog.  and   its   immediate  allies,  the 

"Wolves  and  Foxes. 
The  OEluroidea,  or  cat-like  Carnivora,  comprise  the  Yiven-idaj  (Civets), 

Hya3nas,  and  Cats. 
The   Arctoidea,   or   bear-like  Carnivora,   comprise   the   Mustelidaj 

(Weasels),  Procyonidaj  (Eacoons),  and  the  true  Bears. 

The  order  Carnivora  is  a  very  natural  one,  and  its  name  is.  upon 
the  whole,  fairly  descriptive  of  the  habits  of  the  majority  of  its 
members  ;  though  there  are  some  creatures  included  in  it  which  are 
mixed  feeders,  and  others  which  are  purely  vegetarian. 

In  carnivorous  animals  one  tooth  on  each  side  of  both 
upjDer  and  lower  jaws  is  of  considerable  length,  is  sharply 
pointed,  and  is  called  a  canine ;  the  upper  canine  is  sepa- 
rated by   an  interval   from  the  incisors,   the  lower  canine 


THE    TEETH   OF   CAIINIVUIIA.  375- 


being  received  into  the  vacant  space  or  "  diastema "  so 
formed. 

The  incisors  are  short,  almost  always  six  in  number,  and 
stand  nearly  in  a  straight  line,  transversely  across  the  front 
of  the  jaw,  the  outermost  upper  incisor  being  sometimees- 
large  and  pointed  so  as  to  be  like  a  small  canine. 

The  incisors  and  canines  may,  on  the  whole,  be  said  to  be 
tolerably  uniform  throughout  the  order,  but  the  variations 
in  the  premolar  and  molar  teeth  are  both  numerous  and 
interesting. 

In  the  most  purely  carnivorous  members  of  the  order^ 
the  Felidce,  the  true  molars  are  reduced  to  a  minimum,  and 
the  back  teeth  ai'e  thin  edged,  "  sectorial "  teeth ;  in  the 


bears,  on  the  otlier  hand,  some  of  which  are  purely  her- 
bivorous, the  molars  are  little  short  of  the  full  typical  mam- 
malian number,  and  are  furnished  with  obtuse  and  broad 
gi'iuding  surfaces. 

The  accompanying  figi;re  will  serve  to  give  the  general 
aspect   of  the  teeth    and   jaws    of   a   typically  carnivorous 

{')  Side  view  of  the  cranium  of  a  Tiger,  with  the  mouth  slightly  opened 
to  show  the  relative  position  of  the  great  canines. 


376  A    MANUAL    OF  DENTAL    ANATOMY. 

animiil,  and  to  show  the  great  development  of  the  processes 
for  the  attachment  of  muscles,  and  the  stout  wide  arch  of 
the  zygoma. 

To  a  particular  tooth  in  the  upper  jaw,  and  to  its  antago- 
nist in  the  lower  jaw,  Cuvier  gave  the  name  of  "  carnassial ;" 
these,  conspicuous  in  the  true  flesh-feeders,  become  less  dif- 
ferentiated in  the  Arctoidea  or  bear-like  Carnivora,  and  in 
the  bears  themselves  are  indistinguishable  from  the  other 
teeth,  save  by  a  determination  of  their  homologies  by  a 
process  of  compai'ison  with  the  teeth  of  intermediate  forms. 

The  sectorial  or  carnassial  tooth  in  the  upper  jaw  is 
always  the  fourth  premolar ;  its  crown  is  divisible  into  two 
parts,  the  one  a  thin  sharp-edged  blade,  which  runs  in  an 
iuitero-posterior  direction,  and  is  more  or  less  divided  by  one 
or  two  notches  into  a  corresponding  number  of  cusps ;  the 
other  part,  the  "tubercle,"  is  a  shorter  and  blunter  cusp, 
situated  to  the  inner  side  of  the  anterior  end  of  the  blade 
(see  fig.  16G).  In  those  which  arc  most  purely  flesh-feeders, 
the  "  blade "  is  well  developed,  and  the  tubercle  of  small 
size ;  an  increase  in  the  tul)ercular  character  of  the  tooth  is 
traceable  through  those  genera  which  are  mixed  feeders. 

The  lower  tooth  which  antagonises  the  upper  carnassial, 
passing  a  little  behind  it,  is  the  first  true  molar;  in  the 
Felidce  it  consists  solely  of  the  blade,  which  is  divided  into 
two  large  cusps,  behind  which  is  a  very  small  and  rudimen- 
tary third  division  (which  in  the  Hya;nidce,  for  example,  is 
of  conspicuous  dimensions).  In  existing  Carnivora  but  one 
"  sectorial "  tooth  is  to  be  found  on  each  side  of  the  jaws,  but 
in  the  Hysenodon  and  some  other  extinct  tertiary  mannuals 
there  were  three  teeth  partaking  of  this  character. 

In  a  general  sense  we  may  say  that  the  characters  which 
indicate  a  pure  flesh  diet  are  :  the  small  size  of  the  incisors 
as  compared  with  the  canines,  and  their  arrangement  in  a 
straight  line  across  the  jaw ;  the  large  size,  deep  implanta- 
tion, and  wide  separation  from  one  another  of  the  canines  ; 


THE    TEETH   OF    CARNIVORA.  377 

the  reduction  in  number  of  the  molar  scries,  those  that 
remcain  being  without  broad  crushing  surfaces,  in  the  phacc 
of  which  a  pointed  or  sharp-edged  form  prevails. 

Thus  the  more  numerous  the  teeth  of  the  molar  series, 
and  the  broader  their  crowns,  the  more  likely  it  is  that  the 
creature  subsists  upon  a  mixed  diet ;  and  a  gradation  may 
be  traced  even  in  individual  teeth,  such  as  the  carnassials, 
in  which  a  gradual  increase  in  relative  size  of  the  internal 
tubercular  cusps  of  the  upper,  and  of  the  posterior  tubercles 
of  the  lower  teeth,  may  be  traced  as  we  pass  from  the  ex- 
amination of  the  teeth  of  Felidce,  to  those  of  mixed  feeders, 
such  as  the  Arctoidea. 

It  is  a  familiar  observation  that  immature  animals  differ 
less  from  their  allies  than  do  the  respective  adults,  and  this 
is  exemplified  by  the  milk  dentition  of  the  present  order. 

With  the  exception  of  the  Felidce,  which  have  only  two 
lower  milk  molars,  the  teiTestrial  caruivora,  so  far  as  is 
known,  all  have  the  same  milk  dentition. 

.31        .3 

1  _  c   _  m  -  . 

3        13 

Cynoidea. — The  dog  presents  almost  the  full  typical 
number  of  teeth,  one  upper  molar  (present  in  an  extinct 
dog-like  animal,  the  Amphicyon)  alone  being  wanting. 

.31  4         2 

1  _  c  -    pm  _  m    _  . 

3  1^-1:  3 

The  incisors  are  small,  the  outermost  being  the  largest ; 
the  upper  incisors  have,  as  in  a  great  many  Caniivora,  a 
tri-lobed  shape,  the  surface  of  the  crown  being  marked  by  a 
transverse  groove  into  which  the  apex  of  the  lower  tooth 
fits,  and  the  anterior  of  the  lobes  thus  formed  being  notched 
so  as  to  divide  it  into  two. 

The  canines,  large  and  conical,  are  somewhat  compressed 


378 


A    MANUAL    OF    DEXTAL    AXATOMY. 


from  side  to  side,  and  have  an  anterior  and  a  posterior 
sharp  ridge ;  they  arc  also  slightly  flattened  on  their  inner 
surfaces. 

The  premolars  are  flattened  from  side  to  side,  pointed, 


Fig.  1(3:3  (•). 


#2Vr«i  .b'z^c 


Fig.  164  (-) 


r>  p'  / 


V^v  ^A  /•* 


\j 


increasing  in  size  from  before  backwards,  and  have  small 
basal  accessory  cusps  (see  fig.  163).  The  fourth  upper  pre- 
molar is  the  sectorial  tooth,  and  is  very  much  larger  than 
the  third  premolar ;  the  blade  is  well  pronounced,  and  the 

(')  Dentition  of  Australian  Dog  (Canis  dingo). 

('-)  Milk  and  permanent  teeth  of  Dog  (after  Prof.  Flower). 


THE    TEETH    OF    CAENIVOUA.  37& 

tubercle  small.  The  fourth  lower  premolar  does  not 
greatly  (lifter  from  the  third.  The  two  upper  true  molars 
are  blunt,  broad-crowned  tuberculated  teeth,  but  the  second 
is  very  small. 

In  the  lower  jaw  the  first  true  molar  or  carnassial  tooth, 
lias  a  well-marked  blade,  which  ai-ticulates  with  the  blade 
of  the  upper  carnassial  tooth ;  but  towards  the  posterior 
border  there  is  a  somewhat  thick  and  blunt  tuberculate 
portion,  barely  represented  in  the  corresponding  tooth  of 
the  Felidce ;  the  tubercular  jiortion  articulates  with  the 
bi'oad  flat  first  upper  molar.  The  second  lower  molar  is 
smaller,  not  being  one-fourth  the  size  of  the  first ;  the  third 
smaller  still;  both  are  blunt-crowned  tuberculated  teeth 
(the  third  lower  molar,  rudimentary  in  all  dogs,  is  alto- 
gether absent  in  the  Canis  primsevus). 

The  dentition  of  the  dog,  closely  similar  as  it  is  to  that 
of  the  wolves  and  foxes,  is  such  as  to  allow  of  a  considerable 
i-ange  of  diet,  there  being  tubercular  molar  teeth  in  addition 
to  a  full  armament  of  such  sharply-pointed  teeth  as  are 
characteristic  of  flesh-feeding  animals. 

Thus  the  Canidce,  uniform  as  they  are  in  dentition,  have 
somewhat  different  habits ;  the  Arctic  fox,  a  flesh-feeder 
purely,  has  a  dentition  indistinguishable  from  the  North 
Italian  fox,  which  is  reputed  to  be  vegetarian  in  its  diet ; 
the  Canis  cancrivorus  of  Guiana  eats  small  mammals,  crabs, 
and  also  fruit.  Hence  it  is  necessary  to  be  very  careful  in 
deducing  from  the  character  of  the  teeth  what  may  pro- 
bably have  been  the  diet  of  the  animal  ;  an  approxi- 
mate idea  may  often  be  reached,  but  the  sources  of 
fallacy  are  sufficiently  numerous  to  render  the  conclusion 
uncei-tain. 

Amongst  the  various  breeds  of  dogs  some  slight  differ- 
ences exist.  Thus  in  the  long-muzzled  races  considerable 
intervals  exist  between  the  premolars,  as  is  to  some  extent 
seen  in  C.  Dingo  (fig.  163),  while  in  the  short-muzzled  races 


380  A    MANUAL    OF   DENTAL    ANATOMY. 

the  teeth  are  in  contact,  and  set  somewhat  obliquely,  so  as 
to  be  almost  imbricated. 

On  the  whole  it  may  be  said  that  the  teeth  are  less  easily 
susceptible  of  modification  in  size  than  are  the  jaws,  so  that 
crowding  of  the  teeth  is  induced  by  selective  breeding  aiming 
at  the  production  of  short-nuizzled  varieties. 

In  some  long-muzzled  races  supernumerary  teeth  arc 
sometimes  found ;  thus  De  Blainville  (Ost^ographie, 
Canidce)  figures  two  examples,  the  supernumerary  tooth 
being  in  one  case  a  premolar,  in  the  other  a  true  molar. 

CEluroidea. — With  a  dental  formula  not  differing  much 
from  the  dog  (and  not  all  from  Canis  primpovus)  the 
ViverridcB  (Civet  cats,  Ichneumons,  &c.)  approach  the  more 
typical  carnivores  in  such  points  as  the  thinner  and  sharper 
blades  of  the  premolar  teeth  and  the  greater  relative  length 
and  sharpness  of  the  canines. 

The  dental  formula  is 

.31        4        2 

1  _  c  -  1)   -  m   -  . 

3       1   W         2 

At  the  same  time  the  lower  carnassial  tooth  has  no  less 
than  six  sharply  pointed  cusps,  and  it  lacks  the  typical 
character  of  a  sectorial  tooth,  while  the  long  pointed  cusps 
of  the  molars  of  some  Viverridce  recall  the  characters  of 
insectivorous  dentitions  rather  than  those  of  true  flesh- 
feeders  ;  furthermore,  there  are  other  Viverridcn  which  are 
not  at  all  savage,  and  which  subsist  on  a  diet  of  fruits, 
eggs,  etc.,  such  as  the  Binturong  or  the  Paradoxurus,  the 
teeth  of  which  have  almost  lost  the  carnivorous  character. 
Little  use  can  therefore  be  made  of  the  Viverridce  as  illus- 
trating the  transition  between  the  dental  characters  of  the 
other  families  of  the  order ;  they  rather  serve  to  exemplify 
how,  within  the  limits  of  a  single  family,  with  an  identical 
■dental  formula,  the  form  and  size  of  the  teeth  may  vary  so 


THE    TEETH    OF   CARXIVOEA. 


381 


as  to  adapt  its  members   to   different  forms  of  food  and 
habits  of  life. 

Hycenida'. — lu  the  Hyroua  the  jaw  is  short  and  stont ; 
the  canines  are  set  far  apart,  and  the  teetli  of  tlie  mohir 
series  are  reduced  in  number. 

.314         1 

1  _  c  -  ]i  -  m    -  . 
o       I  ^    \         1 

The  incisors  are  short  and  stout,  but  the  outermost 
upper  incisor  is  somewhat  caniniform ;  the  canines  are  very 
strong,  but  are  not  so  long  relatively  to  the  other  teeth  as 
in  the  Felidce. 

The  premolars  are  all  stout  pointed  teeth,  with  a  very 

Fig.  165  {}). 


well   pronounced  basal   ridge    or   cingulum,  serviceable  in 
protecting   the   gxims   when    the    creature    is    crushing   up 

(')  t-'l'liei-  and  lower  teeth  of  Hy^na.  The  strongly  marked  cingulum 
is  seen  upon  the  lower  teeth.  In  the  upjjer  jaw  the  fourth  premolar 
(carnassial  tootli)  has  a  strong  Made,  divided  into  three  cusps,  and  a 
small  tubercle  opposite  to  and  within  the  anterior  cusp  ;  it  is  a  good 
typical  carnassial  tooth. 


yi    MANUAL    OF   DENTAL   ANATOMY. 


bones ;  they  increase  in  size  from  before  backwards  in  the 
upper  jaw,  tlie  fom-th  npper  premohir  being  a  well  marked 
carnassial  tooth  witti  its  blade  and  tubercle. 

The  lower  cai-nassial  or  first  molar  consists  of  little  more 
than  the  notched  blade  ;  but  the  little  posterior  tubercle  so 
strongly  pronounced  in  the  dog,  is  in  the  hya;na  distinctly 
more  marked  than  in  the  Felidce  (cf  figs.  1G5  and  16G).  The 
only  upper  true  molar  is  the  rudimentary  tooth,  placed 
inside  the  back  of  the  fourth  premolar. 

The  main  featm-e  of  the  dentition  of  the  hysena  is  the 
great  stoutness  and  strength  of  the  teeth ;  they  are  admir- 
ably adapted  to  the  habits  of  the  animal,  which  feeds  rather 
upon  the  portions  of  carcasses  left  by  the  fiercer  carnivora 
than  upon  those  which  it  kills  for  itself,  and  consequently 
bones  form  a  large  proportion  of  its  food. 

There  is  a  curious  hya)na-like  animal  found  at  the  Cape 
(of  which  there  are  often  specimens  at  the  Zoological 
Gardens)  called  Proteles  or  Aardwolf,  in  which  the  teeth 
of  the  molar  series  are  quite  rudimentary.  The  incisors 
(much  worn  in  old  animals)  and  the  canines  are  iaivXy  well 
developed ;  the  molars  and  premolars  quite  stunted. 

The   deciduous   dentition  (  dm.    _.  j  is    similar    to    the 

adult,  as  respects  the  teeth  being  stunted.  It  is  a  cowardly 
animal,  and  is  siipposed  to  feed  on  putrid  flesh ;  it  is  said  to 
eat  young  lambs,  and  to  bite  the  large  tails  of  the  Cape 
sheep,  which  are  remarkal)le  for  containing  an  abundance  of 
semi-fluid  fat, 

Felidce. — The  dentition  of  this  fomily  is  singularly 
uniform. 

.3131 
1  _  c  -  p  -   m  _  . 

Thus  the  molar  series  is  reduced  below  that  of  hysena 


THE    TEETH    OF    CARXIVORA. 


383 


by  the  loss  of  a  premolar  in  both  jaws.  The  iucisors  are 
very  short,  the  cauines  very  large,  -widely  apart,  aud  sharply 
pointed,  with  a  pronounced  longitudinal  ridge  very  charac- 
teristic of  the  Felidce ;  the  premolars  nearest  to  them  are 
quite  short,  so  that  they  stand  practically  alone,  and  so  can 
penetrate  the  flesh  of  living  prey  more  readily. 

The  first  upper  (really  the  second  of  the  typical  mam- 
malian dentition)  premolar  is  almost  a  rudimentary  tooth  ; 


Fio.  166  (1). 


the  second,  a  far  larger  tooth,  is  sharply  pointed ;  the  third 
is  a  well  pronounced  carnassial  tooth,  of  which  the  "  blade  " 
is  divided  by  two  notches  into  three  sharp  lobes,  with  the 
middle  one  of  which  the  "  tubercle  "  is  connected  by  a  slight 
ridge. 

The  solitary  true  molar  is  a  small  touth,  placed  trans- 
versely, aud  within  the  back  of  the  premolar,  so  that  looking 
from  the  outside  it  is  not  visible  at  all. 

In  the  lower  jaw  the  carnassial  (first  molar)  is  reduced  to 

(')  Side  view  of  lower,  and  p;datal  aspect  of  upper  jaw  (Leopard). 


38-1 


A    MANUAL    OF    DENTAL    ANATOMY. 


the  "  blade  "  only ;  it  is  divided  by  a  V-shaped  notch  into 
two  lobes,  and  the  posterior  tubercle  is  hardly  represented. 

In  an  extinct  feline  animal,  the  ^lachairodus,  found  in 
tertiary  strata,  and  very  widely  distributed  (in  France, 
Italy,  India,  Brazil,  Buenos  Ayres)  the  first  of  the  pre^ 
molars  left  in  the  upper  jaw  of  Felis,  and  there  almost 
rudimentary  (see  fig.  166),  has  disappeared;  the  dental 
formula  is  thus  : 


.   3 


The  upper  canines  are  of  immense  length,  and  the  ridge 
of  enamel  which  runs  down  the  front  and  back  surface  of 
the  teeth  is  distinctly  serrated ;  hence  the  name  of  saw- 
toothed  Tiger  which  has  been  given  to  the  animal. 

Fig.  167  ('). 


The  lower  canines  were  quite  small,  and  ranged  with  the 
incisors.  The  enoi'mous  length  of  the  upper  canine  renders 
it  difficult  to  sec  in  what  manner  it  was  made  use  of,  as  the 


{})  Side  view  of  the  jaws  and  cranium  of  Macliairodus  (Drepanodon) 
after  Owen. 


TEE    TEETH    OF    CAnXIVOllA.  SSf. 


mouth  could  hardly  have  been  opened  to  an  extent  sufficient 
to  enable  its  point  to  do  more  than  clear  the  lower  jaw. 

The  extinct  Hyajnodon  had  feline  affinities,  but  diffi)red 
in  that  it  presented  the  typical  mammalian  formula  of 

.3       1       4       3 

1  _    c  -    11   -  m  -  , 
3        1^4       b' 

its  gi-eat  peculiarity  l)eing  tliat  one  and  all  of  these  teeth 
were  of  "carnassial"  form.  Yet  the  elongated  form  of  its 
jaw  is,  so  far  as  it  goes,  opposed  to  the  idea  of  its  having 
been  highly  carnivorous  ;  its  food  at  all  events  must  probably 
have  consisted  of  animals  very  much  smaller  than  itself. 

Arctoidea. — Amongst  the  Carnivora  grouped  together  by 
many  characteristics  as  '  bear-lilce,'  a  tolerably  complete 
gradation  of  character  in  the  matter  of  dentition  may  be 
traced. 

Some  of  the  group,  such  as  the  stoats  and  martins,  are 
very  carnivorous ;  others  are  mainly  herbivorous.  Of  the 
Mustelida)  the  dental  formula  is 

.3        1        4        1 

1  _    c  _   1)    _  m  - 

3       1^4       2 

There  is  a  sort  of  primd  facie  resemblance  to  the  feline 
dentition,  for  the  sectorials  are  very  much  like  those  of  the 
Felidae,  but  the  last  tooth  in  each  jaw  is  a  broad  topped 
tubercular  molar,  even  in  the  most  carnivox-ous  members  of 
the  group,  while  in  those  which  are  less  so,  such  as  the 
badger,  the  molar  teeth  are  very  broad  and  obtuse,  the 
lower  sectorial  having  a  very  small  blade  and  a  very  large 
tubercular  posterior  talon,  so  that,  without  having  really 
lost  its  typical  formation,  it  comes  practically  to  l)e  a  broad 
grinding  tootlu 

In  the   Procyonidtc  (Racoons  and    Coatimundis,  ifcc),  we 

c  c 


A    MANUAL    OF  DENTAL    ANATOMY. 


have  a  further  departure  from  the  carnivorous  character, 
in  the  increased  development  of  the  molar  series  :  the  dental 
formula  is 


3  14  2 
-  c  _  1)  _  m  _ 
3       14       2 


In    the    Coatimundi,    for   cxam^ile,    the   upper   sectorial 


Ai.^iiyjCx^[^ 


has  a  very  large  '  tubei-cle,'  and  posteriorly  to  this  there  is 
a  small  additional  tubercle  ;  the  '  blade '  has  no  large  or 
conspicuous  thin,  flat,  sharp  edge,  but  presents  two  pro- 
nounced cusps. 

The  lower  sectorial  is  no  longer  recognisable  as  a  car- 
nassial  tooth,  but  all  the  true  molars  are  broad  teeth  with 
four  or  five  cusps. 

The  canines  are  very  peculiar,  those  of  the  upper  jaw 
being  very  straight  and  much  flattened  from  side  to  side  ; 
those  of  the  loAvor  jaw  strongly  curved,  and  marked  by  a 
deep  groove  near  the  front  of  their  anterior  surface. 

(')  Upper  and  lower  tooth  of  a  Coatimmuli  (Xasua  socialis).  The  fourth 
upper  premolar  (carnassial  tooth)  has  lost  its  sectorial  character  Ly  tlie 
T)lade  heing  much  less,  and  the  tubercle  much  more  developed  than  in  the 
Qilluroidea  ;  there  is  an  additional  internal  tubercle  at  the  back  of  the 
tooth. 


THE   TEETH    OF   CARXIVOnA. 


387 


In  the  Bears  the  teeth  are  yet  further  modified  to  suit 
the  requirements  of  mixed  or  vegetable  feedei-s. 
The  dental  formula  is  generally 


1 


4  2 
_  m  _ 
4       3 


The  incisors  of  tiie  upper  jaw  present  the  notch  across 
the  crown,  so  common  in  caruivora,  and  the  outermost  is 
large  and  not  iinlike  a  canine;  the  canines  are,  relatively 
to  the  other  teeth,  not  so  large  as  iu  dogs  or  Felidtc ;  never- 
theless they  are  stout  strong  teeth,  upon  which  the  anterior 
and  posterior  ridges  of  enamel  are  well  marked. 

Tlio  first  three  premolars  are  small  dwarfed  teeth;  the 

Fi«.  169  ('). 


first  premolar  is  very  close  to  the  canine,  and  has  a  crown 
of  peculiar  form,  produced  out  towards  the  canine. 

All  four  of  the  premolars  seldom  persist  through  the  life- 


(')  Teeth  of  a  Bear  (Ursus  tluljetanus).  Tlie  figure  is  drawn  from  a 
young  speciiden,  in  which  the  canines  have  liariUy  attained  to  their  full 
length.     In  this  bear  the  fooi-  ijremolars  are  ail  persistent. 


388  A    MANUAL    OF   DENTAL    ANATOMY. 


time  of  the  animal ;  the  first  premolar,  however,  is  rarely 
(if  ever  in  recent  sj^ecies)  lost,  the  second  being  the  first  to 
foil  out,  and  then  the  third.  As  the  fourth  is  never  lost,  in 
most  adult  bears  the  first  and  fourth  premolars  are  foiuid, 
with  a  wide  interval  between  them.  The  i)remolars  of 
bears  thus  form  an  exception-  to  the  rule  that  when  a  tooth 
is  lost  from  the  premolars,  the  loss  takes  idace  from  the 
front  of  the  scries. 

The  fourth  ui.jjcr  premolar  (carnassial  tooth)  retains 
something  of  its  carnassial  character ;  the  first  lower  molar 
very  little,  save  that  it  is  a  narrower  and  more  elongated 
tooth  than  the  other  true  molars. 

The  true  molars  are  squarish  or  oblong  teeth,  raised  into 
blunt  tubercular  cusps  ;  they  vary  in  difierent  species. 

In  the  sloth  bear  (Ursus  labiatus)  the  incisors  are  small 
and  the  median  pair  are  lost  early ;  it  is  varioiisly  stated 
to  be  frugivorous  and  to  feed  on  ants,  the  latter  jirobably 
being  the  more  truthful  account. 

Carxivora  Pinxipedia  (seals). 
The  aquatic  Caniivora  are  divided  into  three  families  : — 

I.  Tlie  Otariida3.  or  Eared  Seals,  comprising-  the  single  genus  Otaria 

known  as  Sea  Lions,  or  Sea  Bears.  These  are  the  "  fur  Seals,'" 
from  which  seal  skin  is  jirocured,  and  they  are  less  removed 
from  tlie  terrestrial  camivora  than  are  the  other  seals  :  the 
limbs  are  better  adapted  for  walking,  there  ai'e  external 
ears,  kc. 

II.  The  Phocidte,   to  which  familj'  the   Seals  of   our  own   coasts 

(Phoca  greenlandica,  kc.)  and  the  Great  Proboscis  Seals  of  the 
southern  seas  (Cystophora)  belong. 

III.  The  Trichechidfe,  or  Walruses,  an  aberrant  Arctic  family  con- 
sisting of  one  genus  only. 

The  dentition  of  the  seals  is  less  highly  specialised  than 
that  of  other  carnivora,  in  some  cases  approximating  to  that 
of  homodont  cetaceans. 

The  canines  are  generally  well  marked  by  being  larger 


THE    TEETH   UF    CARXIVORA. 


than  tlie  other  teeth,  but  the  molars  and  premolars  are 
very  similar  to  each  other,  and  are  simple  in  pattern.  Tlie 
milk  dentition  is  very  feebly  developed  in  the  seals ;  in  the 
Otaria  (fur  seal),  which  of  all  the  seals  most  approaches  to 

Fig.  170  ('). 


the  terrestrial  caniivora  in  other  characters,  the  milk  teeth 
iu-e  retained  for  a  few  weeks,  but  in  most  others  they  arc 


(')  Jaws  of  Otaria,  in  wliicli  tlie  tectli  are  afiectt'cl  !iy  tbe  fona  of  erosion 
alluiletl  to  in  the  text.     After  Dr.  Minie.     Odciit.  Suc.[Traii!^.,  1870. 


300  A    MANUAL    OF   DENTAL    ANATOMY. 

shed  about  the  time  of  birth  (of.  page  300).  Thus  Pro- 
fcsaor  Flower  tells  us  that  in  a  l*hoca  greenlandica  a  week 
old  .scarcely  a  trace  of  the  milk  teeth  was  left. 

The  teeth  of  Otaria  and  of  some  other  seals  become 
much  M'orn  down,  and  they  also  seem  to  become  eroded  at 
the  level  of  the  gums,  as  they  are  often  deeply  excavated  at 
jjoints  which  seem  unlikely  to  have  been  exposed  to  friction, 
but  the  nature  of  this  erosion  has  not  been  adequately  in- 
vestigated. 

The  common  seals  (Phoca)  have  a  dental  formula 

.  n     1     4     1 

1  _   c    _])  _  m  -  . 
;i      1  M      1 

The  incisors  are  of  simple  form,  and  the  outer  are  the 
larger.  The  canine  is  a  strong  recurved  tooth,  with  a 
large  root ;   behind   it   follows  a   series   of  molars,   each   of 

Fifl.  171  ('). 


which  (with  the  exception  of  the  iirst)  bears  a  central 
principal  cusp,  with  a  smaller  accessory  cusp  before  and 
behind  it.  The  forms  of  the  crowns  vary  a  good  deal  in 
different  genera,  in  some  the  cusps  being  much  lai-ger, 
more  deeply  separated  from  one  another  and  recurved ;  and 
in  others  the  accessoiy  cusps  being  multiplied,  so  that  the 

Q)  Teeth  of  Plioca  greenlandica. 


THE    TEETH    OF   CARXIVORA. 


391 


name  of  '  saw-toothed  seal '  has  been    given  to  their  pos- 
sessors. 

In   the  Hooded  seals  (Cjstophora)   the  incisors  are  i*e- 
duced  to  one  in  the  lower  jaw  and  two  in  the  upper ;  the 


Fig.  172  ('). 


-£)jat  iiizo 


canines  are  of  great  size,  hut  the  molars  are  small  and 
simple  in  form,  so  as  to  approximate  to  the  teeth  of  true 
Cctacea. 

The  walrus  (Trichechus  rosmarus),  an  aberrant  Arctic 
form,  is  possessed  of  enormous  upper  canines,  which  pass 
down  outside  the  lower  lip,  and  are  of  such  dimensions  as 
to  materially  modify  the  foi-m  of  cranium  by  the  size  of 
their  sockets ;  they  grow  from  persistent  pulps,  and  are 
composed  of  dentine  with  a  thin  investment  of  cement. 

The  great  tusks  are  employed  to  tear  up  marine  plants 
and  tu  turn  over  obstacles,  the  walrus  feeding  upon  Crus- 
tacea, and  also  upon  seaweed,  &c. ;  they  are  also  used  to 
assist  the  animal  in  clambering  over  ice :  as  they  are  of 
almost  equal  size  in  the  female,  they  cannot  be  regarded  as 
weapons  of  sexual  offence,  but  tliey  are  undoubtedly  used 
in  the  combats  of  the  males. 


")  Permanent  and  milk  teetli  of  Elei)l)aut  Seal  (Cystopboraproboscidea). 


A    MANUAL    OF    DENTAL    ANATOMY. 


In  addition  to  the  great  tusks  the  walrus  ordinai-ily  has  a 
row  of  four  or  five  short  simple  teeth,  worn  down  to  the 
level  of  the  gums  ;  of  these,  the  one  placed  innnediatcly 
within  the  base  of  the  great  canine  is  in  the  inter-maxillaiy 
bone,  and  is  an  incisor  :  the  ordinary  dental  formula  is  given 
by  Professor  Flower  as 


.   1        1 

1   _    c  - 

0        1 


But   there   is   some  difficulty  in  assigni 
Fig.  173  ('). 


a  definite  dental 


formula :  for  in  front  of  the  sulitu)y  incisor  are  often  the 


(')  Siilc  view  of  ujiper  ami  lowei-  jaws  of  a  Waliiis  (Tricheclius  rosmarus). 
The  ujiper  jaw  has  been  tilted  a  little  to  one  side,  in  order  to  bring  into 
view  the  molar  teeth  at  the  same  time  with  the  long  tusks.  The  deter- 
mination of  the  teeth  being  open  to  question,  they  have  been  simply 
umbered. 


THE    TEETH   OF   CAllNIVORA. 


sockets  (or  even  the  teeth  themselves)  or  two  others,  which 
are  for  various  reasons  rather  to  be  regarded  as  noii-per- 
sistcut  teeth  of  the  permanent  set  than  as  milk  teeth  ;  and 
there  are  also  small  teeth  sometimes  to  be  met  with  behind 
the  molars,  which  seem  to  be  rudimentary  permanent  teeth. 
The  teeth  above  alluded  to  may  persist  through  life,  and 
probably  often  do  ;  but  they  arc  sure  to  be  lost  in  macerated 
skulls,  as  they  have  but  little  socket.  Of  the  milk  den- 
tition four  teeth  have  been  traced  in  each  jaw ;  they  are 
rudimentary,  are  lost  about  the  time  of  birth,  and  corres- 
pond in  position  to  the  more  largely  developed  teeth  of  the 
adult.  Hence  tlie  question  if  those  small  rudimentary 
teeth  above  alluded  to  are  to  be  regarded  also  as  milk 
teeth  which  are  long  retained,  or  as  rudimentary  permanent 
teeth  ;  ,it  present  this  requires  further  elucidation. 


CHAPTER    XIII. 

THE    TEETH    OF    IXSECTIVORA,    CHIROrTEi:A,    AND    PRIMATES. 

The  insectivora  fonn  rather  a  heterogeneous  order  of  Mammals. 
aud  embrace  very  various  forms.  All  of  them  are  of  rather  small 
size,  and  some  are  very  small  indeed.  Their  diet  consists  for  the 
most  part  of  insects,  aud  their  teeth  are  generally  adapted  for  this 
by  being  furnished  with  many  points.  The  best  known  animals  in 
the  order  are  the  Hedgehogs,  the  Shrews,  and  the  Moles  ;  to  these 
are  to  be  added  the  Galeopithecus,  or  "  Flying  Lemi^r,"  and  the 
Macroscelidaj  (Elephant  mice).  Insectivora  are  more  abundant  in 
Africa.  Asia,  and  South  America  than  in  Europe.  The  Shrews 
approximate  in  some  measm-e  towards  the  Rodents,  and  the  Tupaia 
is  very  lemurine  in  its  characters. 

The  common  English  Hedgehog  (Erinacens)hasthe  dental 
foimula 

.  3      0         4       3 


In  the  upper  jaw  there  is  a  wide  interval  between  the 
fii'rit  pair  of  incisors,  -which  are  much  the  largest,  and  are 
caniniform  in  shape.  The  next  two  teeth  (incisors)  are 
quite  small,  and  resemble  premolars  in  their  form.  The 
next  tooth  has  two  roots,  and  a  crown  with  one  cusp,  and  is 
also  like  the  premolars  behind  it.  This  tooth,  the  root 
of  which  shows  indications  of  division,  is  sometimes  called 
a  canine,  because  it  comes  next  behind  the  intermaxillary 
suture  ;  behind  this  come  two  small  premolars. 

The  fourth  upper  premolar  is  totally  different  in  size  and 
form   from    the    third  :    its  crown    is    large,   squarish,  and 


THE    TEETH    OF  IXSECTIVOEA.  395 

furnished  with  fo\ir  cusps,  of  which  the  antero-cxtei'nal  one 
is  far  tlie  longest  and  sharpest. 

The  first  upper  true  molar  has  a  square   crown,   upon 
which  are  four  sharp  cusps  :  it  is  implanted  by  four  roots. 

Fig.  17J('). 

.-'        -€^^®)^ 


^ 


V>-  ^..^  -   \rU. 


The  second  true  molar  is  also  square,  quadricusjjid,  and 
luis  four  roots  ;  but  it  is  nuich  smaller  than  the  first,  while 
the  third  upper  true  molar  is  quite  a  small,  compressed, 
double-rooted  tooth,  with  a  thin-edged  crown. 

In  the  lower  jaw  the  first  incisors,  less  widely  separated 
than  the  iipper,  are  also  the  largest ;  then  follows  another 
tooth  termed  incisor,  on  account  of  its  relation  to  the  upper 
incisors  when  the  mouth  is  closed.  The  third  tooth  is 
much  larger,  and  of  peculiar  form.  The  fourth  tooth  from 
the  front  is  a  small  single  tooth,  like  the  third,  but  upon  a 
smaller  scale.  Xext  behind  it,  comes  a  tooth  which  is  very 
much  larger,  and  its  crown  carries  two  principal  cusj^s  with 
a  small  subsidiary  cusp.  The  next  tooth  (first  true  molar) 
has  an  oblong  crown  beset  with  five  sharp  cusj^s,  of  Avhich 
four  are  arranged  at  the  corners  of  a  square,  while  the  fifth, 
obviously  an  elevation  of  the  cingulum,  lies  a  little  in  front 
and  towards  the  inside  of  the  tooth.  In  the  second  true 
molar  the  fifth  cusp  is  but  little  indicated,  while  the  last 

(')   Upper  and  lower  teeth  of  the  Iledgohog. 


A    MANUAL    OF   DEXTAL    ANATOMY. 


true  molar  is  a  dwarfed  tooth  with  but  ouc  cusp.  Several 
dental  formula)  have  been  assigned  to  the  Hedgehog  :  there 
is  little  room  for  difierence  of  opinion  as  to  the  nomen- 
clature of  its  upper  teeth :  though  some  authors  {e.g.. 
Professor  Mivart)  prefer  to  call  the  Hrst  premolar  a  canine. 
But    in    the   lower  jaw    some    :uithors   give  i   -c-pm-, 

others    i   _   c  -  pm  -,  and  othci's  again,  i   -  pm    -.      'I'he 

last  given  seems  the  least  artificial,  and  corresponds  liest 

with  the  relations  between  the  ui)})er  and  lower  teeth  wlien 
the  mouth  is  closed. 

Rousseau   describes    the    existence  of    twenty-four   rnilk 

teeth,  which  he  classifies  thus:   (i  -  dm  _)  :  that  is  to  say, 
'  4  1  ■^ 

all  the  teetli  in  front  of  the  true  molars  had  deciduous  pre- 
decessors, but  his  grouping  of  them  into  incisors  and  molars 
is  quite  arbitrary. 

The  milk  teeth  are  not  shed  and  replaced  until  the 
animal  has  attained  to  almost  its  full  dimensions,  and  all 
three  true  molars  are  in  place. 

The  teeth  of  the  Hedgehog  fairly  represents  some  of  the 
features  of  Insectivorous  dentitions,  for  the  forcep-like  in- 
cisors, the  stunted  or  non -developed  canines,  and  the  molars 
bristling  with  pointed  cusi)S,  are  conmion  to  very  many 
Insectivora. 

The  Sln-ews  have  nimierous  sharply-pointed  teeth,  the 
points  interdigitating  and  fitting  veiy^  closely  together 
when  the  mouth  is  shut.  There  is  no  tooth  either  in  the 
upper  or  lower  jaw  which  is  so  elongated  as  to  deserve  the 
name  of  canine ;  but  between  the  hicisors  and  the  true 
molars  are  several  small  teeth  which,  by  analogy,  are  called 
premolars.  The  true  molars  are  not  very  different  in 
pattern  from  those  of  the  mole  (B  in  Fig.  176),  and  present 
the  W-contour  so  common  in  the  molars  of  Insectivora. 

1'he  most  marked    ])GCuliai-ity    in    the  dentition    of  the 


THE    TEETH    OF   INSECTIVORA.  397 


Shrews  lies  in  the  form  of  their  incisors.  The  first  upper 
iucisor  is  alwuys  very  large  indeed  :  it  looks  vertically 
downwards,  is  a  little  hooked,  and  has  a  notch,  and  a  second 
low  cusp  behind  the  principal  long  pointed  cusp.  The  tip 
of  the  lower  incisor  fits  into  this  notch.  The  lower  incisor 
is  also  very  large  ;  it  lies  nearly  horizontally,  though  the 
point  is  bent  a  little  upwards.  Along  its  upper  edge  there 
are,  in  most  species,  three  or  four  small  cusps,  while  its 
lower  border  is  curiously  prolonged  outside  the  bone  of  the 
jaw,  so  as  to  in  some  measure  encase  this  latter.  The  lower 
iucisor  is  at  least  one-third  as  long  as  the  whole  alveolar 
border.  The  incisor  teeth  of  the  Sln-ew  would  appear  to 
form  a  very  efficient  pair  of  i)incers,  with  which  to  pick  up 
the  minute  creatures  on  which  it  feeds.  Of  the  milk  teeth 
of  Shrews  little  is  known  :  they  are  said  to  be  absorbed 
before  birth,  but  accurate  observations  upon  them  arc  miicli 
needed,  their  very  existence  being  doubtful. 

The  dentition  of  the  Mole  (Talpa)  has  been  the  subject  of 
much  controversy,  the  determination  of  its  canines,  kc, 
presenting  such  difficulty  that  no  less  than  five  different 
dental  formuke  have  been  assigned  to  it. 

In  the  front  of  the  upper  jaw  come  three  small  teeth,  the 
first  being  somewhat  the  largest,  which  are  well  within  the 
limits  of  the  intermaxillary  bone,  and  are  doubtless  incisors. 
But  the  next  tooth,  which  is  very  big,  also  appears  to  be 
implanted  in  the  intermaxillary  bone,  the  suture  passing 
across  its  socket  close,  to  the  back  of  its  posterior  root. 
According  to  its  implantation  it  therefore  would  be  au 
incisor  (')  but  it  is  very  unlike  an  incisor ;  and  it  is  two- 

(')  I  confess  I  cannot  fdllo.v  ilr.  Fpenoe  Bute  when,  in  liis  vahialile 
paper  on  the  milk  teeth  of  tlie  mole,  lie  says,  "  This  tooth  is  implanteil 
within  the  limits  of  the  premaxillary  bones,  the  suture  separating  them 
from  the  maxillary,  passing  through  the  posterior  portion  of  its  alveolus  : 
thus  demonstrdt'uuj  that  this  deciduous  tooth  is  the  true  honiolocjue  of  that 
of  the  mnine  in  the  mammalian  ti/pc."  Surely  it  would  go  to  prove  the 
(.ontrar}',  if  accepted  as  evidence  at  all  upon  this  point. 


398  A    MANUAL    OF    DENTAL    ANATOMY. 

rooted,  a  thing  anomalous  either  in  an  incisor  or  a  canine, 
though  found  in  the  canine  of  (ijnuun-a,  which  is  l)eyond 
(juestion  in  the  maxiUary  bone. 

Next  come  three  minute  premolars,  and  a  fourth,  which 
is  much  larger  than  the  others  :  these  all  have  single  crowns, 
consisting  of  little  more  than  a  single  sharply-pointed  cusp. 

The  first  two  upper  molars  are  large  teeth  bristling  with 
cusps  :  the   third  is  much  reduced   in  size  and    siiiiplifiod 


~  NaV.  Size. 


in  pattern,  in  the  lower  jaw  tiie  four  fruut  teeth  are  all 
small,  but  the  fourth  or  outermost  of  these  incisors  is  called 
by  some  writers  the  lower  canine,  because,  when  the  teeth 
are  closed,  it  passes  in  front  of  the  upper  caniniform  tooth. 

Nevertheless  the  tooth  which  does  the  work  of  a  canine  in 
the  lower  jaw  is  the  fifth  counting  from  the  front  :  this  is  a 
two-rooted  tooth,  and  conforms  so  closely  with  the  thi'ce 
teeth  behind  it  in  configuration,  that  it  is  obviously  only  one 
of  these  premolars  developed  to  a  greater  length  than  the 
others.  It  closes  hehind  the  caniniform  upper  tooth,  so 
cannot  on  this  ground  be  called  a  canine  by  those  who 
attach  importance  to  the  term. 

Q)  Upper  and  lower  teeth  of  the  common  Mole.  The  functionless  milk 
teeth  (after  Spence  Bate)  are  p'aced  above  the  permanent  teeth  which  dis- 
place them. 


THE   TEETH    OF   IXSECTIVORA.  399 


The  remcaiuing  three  premolars  are  rather  small  and 
single ;  the  true  molars  are  of  considerable  size,  and  their 
points  are  very  long  and  sharp. 

I  have  purposely  avoided  giving  any  dental  formula  for 
the  Mole :  everything  turns  upon  the  value  which  we 
attach  to  the  term  canine  ;  and  I  have  already  given  reasons 
(p.  284r)  for  attaching  but  little  homological  importance  to 
its  determination. 

Mr.  Spence  Bate's  paper  (Trans.  Odontol.  Society,  1867), 
valuable  as  it  is  in  contribiiting  to  our  whole  knowledge  of 
the  milk  dentition  of  the  creature,  does  not  appear  to  me  to 
help  us  much  in  the  matter  of  determining  the  homologies 
of  the  canine. 

In  a  Mole  3|  inches  long  he  found  eight  milk  teeth  on 
each  side  of  both  upper  and  lower  jaws,  as  is  indicated  in 
Fig.  175.  The  milk  incisors  were  about  one-twentieth  of 
an  inch  in  length,  :uid  one  two-hundredth  in  diameter,  and 
were  rudimentary  in  form,  consisting  of  long  thin  c\'lindrical 
tubes  surmounted  by  slightly  expanded  crowns.  All  the 
milk  teeth  were  of  this  simple  form,  save  only  the  last  in 
each  jaw,  which  presented  crowns  with  two  cusps,  and  had 
their  roots  to  some  little  extent  divided  into  two. 

At  the  time  when  these  teeth  are  present  the  intermaxil- 
lary suture  is  very  distinct,  and  there  is  no  doubt  that  the 
fom-th  upper  milk  tooth,  the  predecessor  of  the  caniniform 
tooth,  is  in  the  intermaxillary  bone. 

The  teeth  had  not  fairly  cut  the  gum,  and  the  advanced 
state  of  the  permanent  teeth  beneath  them  make  it  doubtful 
whether  they  ever  do  become  erupted.  At  all  events,  they 
can  be  of  no  use. 

In  many  of  the  order  Insectivora  the  milk  dentition  is 
xmkuown,  but  we  have  exemplified  amongst  them  cveiy 
grade  of  completeness  in  its  development.  Thus  in  the 
Hedgehog  and  Ccntctos  (an  allic>l  animal  from  Madagascar) 


//    MANUAL    OF   UENTAL    ANATOMY. 


the  milk  dentition  is  tolerably  complete,  while  in  the 
Shrews  it  has  all  bnt,  or  quite,  disappeared. 

The  W-patteni  characterising  the  molars  of  Insectivora 
has  already  been  alluded  to ;  it  is  well  exemplified  in  the 
molar  of  Urotrichus. 

In  this  tooth,  as  has  been  clearly  shown  by  Prof.  Mivart 
(Osteology  of  Insectivoi-a,  Joimi.  of  Anat.,  1868),  the  four 
cusps  of  the  typical  teeth  [a,  h,  c,  d)  have  been  added  to 
by  the  elevation  of  the  cingulum  into  three  or  four  external, 
and  one  internal  cusp,  making  up  the  total  number  to  nine. 
Thus  it  is  that  the  molars  of  this  order  often  fairly  bristle 
with  cusps. 

In  the  Mule  the  number  of  cusps  is  diminished  by  the 
coalescence  of  h  and  d  into  a  ridge,  and  tlic  disappearance  of 
Fig.  n<M'»- 

^  .. .  ^i     , 


AFC 
the  inner  cusp  of  the  cingulum,  Avhile  the  simplification  is 
carried  yet  farther  in  the  Cape  Mole  (c  in  I'^ig.  176). 

It  would  be  impossible  to  notice  the  somewhat  varied 
dentitions  of  other  Insectivora  in  these  pages,  but  mention 
must  be  made  of  the  very  anomalous  teeth  of  the  Galeo- 
pithecus,  formerly  placed  with  the  Lemurs  under  the  title  of 
"Flying  Lemur." 

Its  lower  incisors  are  divided  by  a  number  of  vertical 
divisions  running  down  through  a  great  part  of  the  length 
of  the  crowns,  so  that  they  can  be  compared  to  combs,  or  to 

(')  A.  Upper  molar  of  Urotricluis  ;  I!.  Jtolc  :  C.  Cape  Iridescent  Mole, 
(Chrysochloris). 


THE    TEETH    OF   INSECTIVORA. 


hands  with  the  fingers  slightly  separated.  What  the  pur- 
pose served  by  these  comb-like  teeth  ma}^  be  remains 
nncertaiu  :  no  other  animal  has  similar  teeth.  (Jaleopithe- 
cus  has  a  well  developed  milk  dentition,  the  milk  teeth 
being  very  similar  to  their  successors. 

The  teeth  of  Insectivora  are  remai-kable  for  the  thickness 
of  their  enamel,  which  in  the  Shrews  is  to  some  extent 
penetrated  by  the  dentinal  tubes.  The  enamel  is  deeply 
coloured  in  some  Shrews,  the  pigment  being  actually  in  the 
substance  of  the  enamel,  and  not  in  any  distinct  layer. 

THE    TEETH    OF    CHIROPTEltA. 

The  Bats,  sharply  distinguished  fi-om  all  other  mammals 
by  the  possession  of  wings,  are  divided  into  two  groups, 
respectively  insectivorous  and  frugivorous. 

The  insectivorous  Bats,  by  far  the  most  numerous  section, 
are  for  the  most  part  possessed  of  small  incisors,  rather 
large  canines,  and  premolar  and  molar  teeth  which  bristle 
with  shai-p  cusps,  and  generally  present  the  W-pattcrn.  In 
fact,  in  general  character,  their  teeth  resemble  those  of  the 
Insectivora. 

The  incisors  are  sometimes  reduced  in  uumbei-,  and 
spaces  left  betvreen  them ;  and  some,  as  for  example,  the 
Vampire  (Desmodus)  have  teeth  specially  modified  to  accord 
with  their  blood-sucking  habits. 

This  Bat  has  only  one  permanent  incisor  on  each  side,  and 
this  is  a  lai'ge  but  thin  and  sharp-edged  tooth,  with  which  the 
wound  is  made  ;  the  lower  incisors  are  small  teeth  with  feebly 
notched  edges.  The  canines  are  large,  and  the  molar 
series,  which  is  not  required  in  an  animal  existing  upon 
blood,  is  stunted.  The  molar  teeth  are,  however,  sharp, 
though  small,  and  there  is  no  marked  distinction  into 
molars  and  premolars. 

The   frugivorous  bats  (of  which  the  Pteropus,  or  flying 

D  D 


402  A    MANUAL    OF   DENTAL    ANATOMY. 


fox,  is  an  example)  have  much  Larger  muzzles,  and  the  molar 

teeth  are  set  with  intervals  between  them. 

2     12       3 
The  dental  formula  is  i  -  c  -  p  -  m  -,    hut    in  some  the 
2     1  ^  3       3' 

molar  scries  is  reduced  below  tliis  number. 

The  incisors  are  small,  and  the  canines  rather  hu-gc. 

Both  molars  and  premolars  are  of  somewhat  simple  form, 
being  long,  and  compressed  from  side  to  side.  The  outer 
borders  of  the  crown  of  the  molars  are  elevated  into  distinct 
but  not  exceedingly  sharp  cusps,  which  become  worn  down 
by  use. 

The  insectivorous  character  of  the  presence  of  many 
sharp  cusps  upon  the  teeth  is  not  to  be  found  in  any  of  the 
frugivorous  bats.  All  the  Pteropi  have  deciduous  canines, 
and  four  deciduous  molars,  of  simple  pointed  form,  but  the 
number  of  deciduous  incisors  is  very  variable. 

The  milk  dentition  of  bats  has  been  very  carefully  and 
thoroughly  investigated  by  Leche  (Lund's  Universit.  Ars- 
skrift,  Tom.  XII.  and  XIV.,  1878),  and  at  the  present  the 
Megadermata  are  the  only  family  in  which  the  milk  teeth 
are  unknown.  The  milk  teeth  are  not  of  much  functional 
importance,  as  they  are  shed  soon  after,  if  not  absorbed 
before,  birth,  and  they  are  not  therefore  implanted  in  very- 
definite  sockets. 

In  their  slight  cylindrical  elongated  roots,  surmounted  by 
expanded  crowns,  these  milk  teeth  often  recall  those  of  the 
Mole. 

Sometimes  the  milk  teeth  are  to  be  found  even  after  the 
permanent  teeth  are  in  situ;  in  other  instances,  as  for 
example  the  deciduous  molars  of  Molossus,  they  never  cut 
the  gum.     The  milk  dentition  of  the  Vampire  (Desmodus)  (•) 

(')  In  a  skull  of  Desmodus,  in  the  ijossession  of  Air.  R.  F.  Tomes,  the 
third  milk  tooth  appears  to  correspond  in  position  to  the  iiernianent 
canine  ;  the  same  is  the  case  in  the  specimen  iigured  by  Messrs.  Gervais 
and  Castelmain  (Expcd.  dans  les  part.  cent.  d'Amerique  du  Sud). 


THE    TEETH    OF   FIUMATES.  403 


appears  to  consist  of  incisors  only,  or  of  incisors  and  canines ; 
though  the  absence  of  observed  molars  may  be  due  to  the 
fact  that  they  are,  as  in  Molossus,  shed  very  early. 

It  has,,  near  to  the  fi-ont  of  the  upper  jaw,  six  teeth,  each 
of  which  is  very  long  and  slender,  and  has  a  strongly  hooked 
point :  it  has  been  suggested  that  these  feeble  hooked  teeth 
may  assist  it  in  holding  on  to  the  mother. 


THE    TEETU    OF    PRIMATES. 

The  order  Primates  embraces  Man,  Monkeys,  and  the 
Lemm-s. 

Some  naturalists  have  been  disposed  to  separate  the  Lemuridfe 
from  the  rest  of  the  Primates,  on  the  ground  that  some  Lemurs 
approximate  rather  closely  to  the  Insectivora.  while  again  the  order 
Insectivora  contains  some  forms  which  recall  the  Lemurs. 

But  although  the  Lemm-ida;  are  undoubtedly  inferior  to  the 
Monkeys,  and  stand  apart  from  them  more  widely  than  do  the 
Monkeys  from  Man.  most  authors  now  place  them  in  the  order 
Primates,  which  is  to  be  divided  as  follows  : — 

I  Lemuridiv.  Lemurs. 

Primates   <,'  Simiada;.  Old  and  new  world  Monkeys. 

( AnthropidaB.         Man. 

Isemuridse. — The  Lemurs  for  the  most  part  are  foimd  in 
Madagascar,  and  to  a  less  extent  on  the  mainland  of  Africa 
and  in  southern  Asia.  In  their  dentition,  just  as  in  other 
characters,  they  ditter  somewhat  from  the  true  monkeys, 
though,  on  account  of  there  being  several  very  aben-ant  in 
form,  it  is  difficult  to  give  any  general  account  of  them. 
Most  of  them  have  the  upper  incisors  very  small,  and  widely 
separated  from  one  another ;  in  the  lower  jaw  these  are 
antagonised  by  six  long,  thin,  narrow  procumbent  teeth, 
generally  regai-ded  as  being  two  pairs  of  incisors  and  the  lower 
canines:  in  both  upper  and  lower  jaws  the  next  tooth  is 
large  aud  pointed  like  a  canine,  but  the  lower  caniniform 
tooth  bites  behind  the  upper,  and  therefore  is  held  not  to 


404  A    MANUAL    OF   DENTAL    ANATOMY. 

corrcsjiond  to  it,  but  to  be  the  first  jn-emolar  (of.  page  283). 
The  premolars  are  compressed  from  side  to  side,  and  are 
very  sharp  :  the  molars  are  armed  with  long  sharp  cusps, 
which  are  worn  down  in  old  animals. 

The  upper  molai-s  iu  many  lemm-s  are  armed  with  four 
cusps,  connected  by  an  "oblique  ridge"  like  those  of  man 
and  the  anthropoid  apes. 

There  is  a  very  aberrant  lemur,  the  x^ye-aye  ((Jheiromys), 
which  in  its  dentition  imitates  the  rodents. 

.10  1        3 

1  _  c  -  pm  _  m  -  • 

In  both  upper  and  lower  jaws  the  incisors  form  a  single 
pair  of  large  curved  teeth,  growing  from  persistent  pulps, 
and  wearing  obliquely  so  as  to  constantly  preserve  a  sharp 
cutting  edge.  The  enamel  is  very  much  less  thick,  if  not 
idtogether  absent,  upon  the  backs  of  these  incisors. 

After  a  considerable  interval,  which  is  devoid  of  teeth, 
there  follow  four  upper  and  three  lower  teeth,  Avhich  are 
not  of  persistent  growth,  but  have  definite  roots,  and  re- 
semble the  molars  of  many  omnivorous  rodents. 

Being  a  somewhat  rare  and  strictly  nocturnal  animal, 
little  is  known  of  its  food  ;  some  have  believed  that  it  made 
use  of  its  rodent  incisors  to  cut  away  portions  of  wood  in 
order  to  get  at  the  grubs  contained  in  it,  drawing  them  out 
of  their  hiding  place  by  means  of  its  curiously  elongated 
finger,  whilst  others  believe  that  it  gnaws  the  sugar  cane. 
But  whatever  the  nature  of  its  food  may  be,  it  is  certain 
that  its  scalpriform  incisors  are  put  to  hard  work,  and  so 
kept  worn  down,  for  in  a  specimen  kejit  for  a  time  in  the 
Zoological  Gardens,  which  was  supplied  wnth  soft  food, 
the  incisor  teeth  grew  to  an  excessive  length,  and  idtimately 
caused  the  animal's  death  by  the  points  of  its  lower  incisors 
perforating  the  palate.  The  accompanying  figure  represents 
the  muzzle  of  this  specimen,  and  altho\igh  the  upper  teeth 


THE    TEETH    OF   PRIMATES. 


have  grown  to  an  inordinate  length,  and  have  divei'ged  frora 
one  another,  it  will  serve  to  show  the  rodent-like  aspect  of 
its  mouth. 

Although,  functionally,  its  teetli   are  those  of  a  rodent, 
yet  despite  this  adaptive  rcsemMance,  the  milk  dentition 

Fig.  177('). 


retains  certain  characteis  which  indicate  the  Icmuriue  origin 
of  the  creature. 

In  the  upper  jaw  the  milk  dentition  consists  of  two  small 
incisors,  a  canine  and  three  molars;  in  the  lower  jaw  of  two 
small  incisors  and  two  small  molars ;  it  is  said  that  in  an 
early  stage  a  third  milk  incisor  is  to  bo  found. 

The  permanent  incisors  push  their  way  up  between  the 
first  and  second  milk  incisors  ;  at  a  certain  stage  all  three 


(';  Aye- Aye  (Cheiromys),  which  died  in  the  Zoological  Gardens  (after 
Dr.  Murie).  The  upper  incisors,  from  want  of  sufficient  use,  have  grown 
long  and  diverged  from  the  middle  line. 


406 


A    MANUAL    OF  DENTAL    ANATOMY. 


are  to  be  seen  at  once,  but  the  large  size  of  the  permanent 
incisors  causes  the  speedy  loss  of  the  milk  incisors. 

No  known  rodent  has  so  many  milk  teeth,  nor  indeed  any 

Fig.  1/8(1). 


milk  incisors  at  all ;  the  aye-aye  thus  affords  an  excellent 
example  of  a  milk  dentition  presen-ing  characters  which  arc 
the  extremely  modified  adult  dentition. 


lost 


The  special  interest  which  attaches  to  the  dentition  of 
Cheiromys  has  been  already  alluded  to  (see  page  271)  ;  to 
briefly  recapitulate,  it  is  this  :  in  Madagascar,  an  isolated 
area  separated  by  a  wide  tract  of  deep  sea  from  other  areas, 
true  rodents  are  almost  absent,  but  lemurs  abundant.     But 


(1)  Upper  and  lower  jaws  of  Cheiromys.  A.  Milk  dentition,  witli  the 
permanent  incisors  jnst  emerging,  i,  I.  Upper  and  lower  permanent  in- 
cisors. i2,  12.  Upper  and  lower  milk  incisors,  c.  Milk  canines,  rfl, 
d  2,  d  a,  d  b.  Upper  and  lower  milk  molars.  (Twice  natural  size.)  B. 
Reduced  figiu-e  of  permanent  teeth  (after  Peters). 


THE    TEETH    OF   PllUIATES.  407 

one  of  the  lemiirine  animals  which  are  to  be  found  there  has 
been  so  modified  that  its  teeth  to  all  intents  and  purposes 
are  those  of  a  rodent.  Yet  with  all  this  modification  it 
retains  characters  (notably  its  milk  dentition)  -which  are 
quite  unlike  those  of  true  rodents,  but  which  recall  the 
manner  of  its  origin  from  higher  lemurine  fonns. 

Simiadse. — The  true  monkeys  are  divided  into  two 
great  divisions,  the  new  world  monkeys  and  the  old  world 
monkeys.  The  former  difter  in  many  respects  from  the 
latter;  for  the  most  part  they  have  prehensile  tails,  and 
their  nostrils  are  set  somewhat  wndely  apart,  whence 
they  are  called  Platijrrhine,  or  wide-nosed  monkeys,  and 
they  differ  also  in  their  dental  formula,  which  is — 

.2     1     3       3     .,„ 
1  _  c  -  p  -  m  -  =  oo. 
2     1^3       3 

The  little  marmoset  monkeys  have  only  32  teeth,  but  they 
agree  with  the  other  new  world  monkeys  in  having  three 
pi'emolars  on  each  side,  the  molars  being  reduced  to  two  in 
number.  The  upper  molars  of  many  new  world  monkeys 
have  the  antero-interual  and  extcro-posterior  cusps  joined 
by  an  oblique  ridge,  a  character  which  is  shared  in  the  old 
world  groups  by  man  and  the  anthropoid  apes  only. 

All  the  Quadrumana  have  a  well  developed  milk  den- 
tition. 

Old  world  or  (Jaturrhine  monkeys  all  have  the  same 
dental  formula  as  man — 

*>     1     2      3 
i^c-p_m-; 

As  an  example  the  Macaque  monkey  may  be  taken.  The 
upper  and  lower  incisors,  but  especially  the  former,  are 
directed  obliquely  forwards,  and  the  lateral  incisors  are  very 
much  smaller  than  the  centrals.  In  the  upper  jaw  a  con- 
siderable interval  separates  the  incisors  from  the  canine, 


408 


A    MANUAL    OF  DENTAL    ANATOMY. 


whicli  is  a  veiy  large  tooth,  somewhat  triangular  in  section, 
with  a  sharp  edge  directed  l)ackwards,  and  with  a  deep 
groove  on  its  anterior  surface. 

The  upper  premolars  are  implanted  by  three  distinct 
roots,  as  arc  also  the  true  molars  ;  the  latter  are  quadri- 
cuspid,  but  lack  the  oblique  ridge. 


Nal.  S'fzc 


The  lower  canine  is  a  sharp  and  powerful  tooth,  though 
it  is  very  much  smaller  than  the  upper ;  the  first  lower 
premolar,  by  its  front  surface,  articulates  with  the  upper 
canine,  and  is  of  curious  form.  It  is  implanted  by  two 
roots,  but  the  anterior  root  is  produced  forwards,  so  that  the 
antero-posterior  extent  of  the  tooth  is  much  increased. 

The  apex  of  the  cusp  of  the  tooth  is  almost  over  the 
posterior  root,  and  from  this  point  the  crown  of  the  tooth 
slopes  obliquely  forwards  down  to  its  anterior  root.  This 
peculiarity  in  the  form  of  the  first  lower  premolar  is  eminently 

(')  Upper  and  lowor  teeth  of  a  Monkey  (Macacus  neiiiestiinus,  male). 
The  length  and  sharpness  of  the  canines,  and  the  peculiar  form  of  tho 
anterior  lower  premolar,  contrasts  with  the  aspect  of  the  coiresponding 
teeth  in  the  Anthropoid  Apes  or  in  Man. 


THE    TEETH    OF   PIHMATES.  409 


characteristic  of  the  baboons.  There  is  nothing  to  note  of 
the  second  premolar  save  that  it  is  implanted  by  two  roots, 
like  the  true  molars,  which  are  quadricuspid  ;  of  them  the 
thu'd  is  lai'ger  than  the  first  two,  and  is  quinquicuspid. 

There  is  considerable  difterence  in  the  size  of  the  canine 
in  the  two  sexes,  that  of  the  male  being  very  much  the 
larger ;  this  ditterence  does  not  exist  in  the  deciduous  den- 
tition, in  which  the  canines  are  relatively  small. 

The  Anthropoid  Apes  are  the  Gibbons  (Hylobate.s),  the 
Chimpanzee  (Simla  Troglodytes,  or  Troglodytes  niger),  the 
Orang  (Simla  or  Pithccus  Satyius),  and  the  Gorilla  (Troglo- 
dytes Gorilla). 

Upon  the  whole  the  gibbons  are  the  lowest,  and  the 
gorilla  the  highest  of  the  anthropoid  apes,  which  arc  all 
confined  to  tropical  areas.  Thus  the  gorilla  and  chimpanzee 
are  confined  to  tropical  Africa,  and  the  orang  is  limited  to  a 
part  of  the  Malay  archipelago.  The  gibbons  are  more 
widely  distributed  over  the  Malay  archipelago  and  tropical 
Asia. 

Although  upon  the  whole  the  gorilla  approaches  most 
nearly  to  man,  this  can  hardly  be  said  to  be  the  case  Avith 
its  dentition.  The  jaws  are  veiy  square,  and  there  is  a  large 
diastema  in  front  of  the  upjier  canine,  which  in  the  male 
gorilla  is  of  great  size  and  strength,  its  top  descending  far 
below  the  level  of  the  aheolar  border  of  the  lower  jaw  when 
the  mouth  is  shut. 

In  the  lower  jaw  there  is  no  diastema,  but  the  teeth  are 
all  in  contact  with  one  another ;  the  first  of  the  pre-molai-s 
is  a  very  strong  pointed  cone,  showing  plainly  the  close 
relationship  between  canines  and  premolars  alluded  to  at  a 
previous  page  (p.  17). 

The  molars  increase  in  size  from  before  backwards,  the 
third  molars  attaining  to  a  very  large  size. 

Nevertheless,  though  the  teeth  are  coarser  and  stronger, 
there  is  a  general  resemblance  to  those  of  man. 


410 


A    MANUAL    OF  DENTAL    ANATOMY. 


It  has  been  pointed  out  by  the  late  Professor  lioUeston  that 
the  canine  tooth  of  the  male  anthropoid  apes  is  a  little  later 
in  coming  into  place  than  in  the  female.  Thus  in  the  male 
chimpanzee  and  orang,  it  is  not  cut  initil  after  the  third 
molars  (wisdom  teeth)  are  in  place,  whereas  in  the  female 
it  follows  the  second,  but  precedes  the  third  molars.  The 
sexual  difference  in  the  canine  teeth  is  very  well  marked  in 
all  the  anthropoid  apes,  and  its  later  eruption  in  the  males 


sxVol  Si-f 


is  explicable  both  upon  the  ground  that,  being  a  sexual 
weapon,  it  is  not  needed  prior  to  the  attainment  of  sexual 
maturity,  and  also  that  being  of  very  large  size  its  formation 
might  be  expected  to  take  a  longer  time.  No  such  difference 
pertains  to  the  milk  dentition,  in  which  the  order  of  eruption 
is  exactly  that  met  with  in  man. 

Dr.  Magitot  (Bulletin  de  la  Societc  d'Anthropologie  de 
Paris,  1869)  combats  the  idea  that  there  is  any  difference  in 
the  order  of  the  eruption  of  the  permanent  teeth  between 
man  and  the  anthropoid  apes,  but,  while  his  observations 
have  been  both  careful  and  widely  extended,  he  lays  much 


(')  Upper  and  lower  teeth   of  an   Antln-(ipoi 
Orang  Outan). 


Ape   (Siniia  Satyi-us,   or 


THE    TEETH    OF  PMIMATES.  411 


stress  upon  an  observation  made  upon  a  female  gorilla  skull, 
in  which,  as  has  just  been  mentioned,  the  order  of  succession 
is  not  quite  the  same  as  in  the  male. 

The  dentition  of  the  orang  approaches  tolo-ably  closely  to 
tliat  of  man,  and  the  points  of  resemblance  and  of  difference 
may  be  fairly  well  seen  in  the  accompanying  figure. 

The  central  upper  incisors  are  similar  to  those  of  man, 
but  are  larger  ;  the  laterals  are,  relatively  to  the  centrals, 
much  smaller,  and  are  very  caniniform  in  shajje,  both  inner 
and  outer  angles  of  their  cutting  edge  being  sloped  off"  to 
such  an  extent  that  a  central  pointed  ciisp  remains,  in 
place  of  a  thin  cutting  edge.  The  canines  are  strong, 
pointed  teeth,  the  cinguluni  and  the  ridge  joining  it  with 
the  apex  of  the  cusp  being  well  marked  upon  their  imrer 
sides.  In  the  female  the  upper  canine  is  about  half  as 
long  again  as  any  of  the  other  teeth ;  in  the  male  it  is 
longer. 

The  first  Incuspid  is  a  little  more  caniniform  than  that 
of  man  :  its  outer  cusp  is  long  and  pointed,  and  a  i-idge 
unites  it  with  the  anterior  part  of  the  inner  cusp,  which  is 
feebly  pronounced;  the  second  is  a  blunter  and  broader 
tooth.  The  premolars  are  implanted  by  three  roots.  The 
molars  are  not  unlike  the  human  teeth  in  pattern. 

In  the  lower  jaw  the  incisors  are  large  and  stout ;  the 
canines  sharply  pointed,  with  a  well  marked  cingulum, 
and  a  well  marked  median  ridge  on  the  inner  side  of  the 
crown.  The  first  premolar  is  a  shorter,  stouter,  and 
lilunter  copy  of  the  canine,  and  can  hardly  be  said  to 
liavc  an  inner  cusp.  In  the  second  premolar  the  inner 
cusp  is  as  high  as  the  outer,  and  the  cingulum  is  elevated 
both  before  and  behind  till  it  almost  forms  two  additional 
cusps. 

Indeed,  [  am  not  acquainted  with  any  dentition  which 
exemplifies  the   transition  from    incisors    to    canines,  from 


412 


A    MANUAL    OF   DENTAL    ANATOMY. 


canines  to   premolars,   and  from  premolars  to  true  molars, 
better  than  that  of  the  oranf'. 


Fig.  181  Q 


Fig.  182  {-). 


Fig.   ]  83  C'}. 


The  lower  molars  resemble  those  of  man,  save  that  their 


(')  Skull  of  a  .young  male  Orang.     The   ujipcr  canine   dues   not  nearly 
reach  to  the  lower  alveolar  border. 

(-)  Skull  of  adult  male  Orang,  in  which  the  canine  is  largely  developed . 
(•')  Side  view  of  skull  of  an  idiot. 


THE    TEETH    OF   PRIMATES. 


surface  is  marked  by  that  finely  wrinkled  pattern  -wliicli  is 
common  to  all  the  unworn  teeth  of  the  orang.  One  is 
struck  b}'-  the  great  backward  elongation  of  tlie  jaws,  by 
their  squareness,  by  the  parallelism  of  the  two  sides 
which  converge  slightly  at  the  back,  and  by  the  large  size 
of  the  teeth  in  proportion  to  the  bulk  of  the  whole  animal. 

The  large  size  of  the  canines  being  in  a  measure  a  sexual 
cliaracter,  is,  as  is  so  often  the  case,  not  very  noticeable  in 
the  young  animal :  the  two  acompanying  illustrations  of 
a  young  and  an  adult  male  orang  may  serve  to  show  this,  as 
well  as  some  other  differences  developed  by  age. 

The  differences  which  serve  to  distinguish  the  dentition 
<jf  the  most  anthropomorphic  apes  from  that  of  man  are 
mainly  these.  Relatively  to  the  size  of  the  cranium,  and  of 
the  whole  creature,  the  teeth  and  jaws  are  very  much  larger 
in  all  their  dimensions ;  hence  the  creatures  are  progna- 
thous, and  the  facial  angle  small,  even  when  compared 
with  the  jaws  and  cranium  of  an  idiot.  As  might  be  ex- 
pected this  difference  is  not  so  great  in  the  yoinig  as  in  the 
adult  animal. 

In  place  of  the  teeth  being  arranged  in  a  swcejiing  curve, 
the  jaws  are  squarish,  the  incisors  being  arranged  in  some- 
thing approaching  to  a  straight  line  between  the  two  great 
outstanding  canines,  behind  which  the  premolar  and  molar 
series  run  in  straight  lines,  converging  somewhat  as  they  go 
backward.  There  is  a  ''  diastema  "  Q)  or  intei-val  in  front  of 
the  ujjper  canine,  into  which  the  point  of  the  lower  canine 
passes,  when  the  mouth  is  closed.  Both  the  greater  square^ 
ness  of  the  jaws,  and  the  existence  of  a  diastema,  are  direct 
results  of  the  gi-eat  size  of  the  canines,  and  are  consequently 
not  marked  in  young  specimens. 

The  upper  premolars  are  implanted  by  three  roots,  the 

{')  Pometliing  approaching  to  a  diastema  is  said  to  liavc  been  observed 
by  Vogt  and  Broca  in  early  Eiu-opean  skulls. 


414 


A    MANUAL    OF  DENTAL    ANATOMY. 


lower  by  two  roots,  just  like  the  true  molars,  and  the  prc- 

Fii:.  IS  I  M). 


.Ti\^./A,S/^. 


molars  when  unworn  partake  more  of  the  pointed  character 
than  they  do  in  man. 

{})  Upper  teeth  of  a  Caffir.  The  oblique  ridge  of  the  upper  molar  i-- 
distinct,  not  only  upon  the  first  and  second,  but  also  upon  the  third  uiolai- 
or  wisdom  tooth,  which  in  this  skull  has  the  normal  three  roots  well 
marked. 

(-)  Lower  jaw  of  a  Caffir,  in  which  the  quinquicuspid  form  of  the  first 
and  third  molars  is  well  seen,  it  being  somewhat  les.s  strongly  indicated  iu 
the  second  molars. 


THE    TEETH    OF    PRIMATES.  415 


The  wisdom  teeth  present  the  same  pattern  of  grinding 
surface,  are  larger  than  the  other  molars  in  the  gorilla 
and  the  orang,  and  thei-e  is  abimdant  space  for  them,  so 
that  they  play  an  important  part  in  mastication.  The 
molar  teeth  of  these  apes  are  also  squarer,  their  cusps 
sharper  and  longer,  and  the  characteristic  patterns  more 
strongly  prouomiced,  than  in  man. 

Anthropidse. — In  passing  from  the  highest  of  the  apes 
to  the  lowest  of  mankind,  there  is  a  sudden  change  in  the 
chai-acter  of  the  dentition ;  but  while  it  cannot  but  bo 
admitted  that  there  is  a  gap,  yet  the  differences  are  rather 
of  degree  than  of  kind. 

Even  in  the  lowest  of  human  races  the  facial  angle  is 
greater,  that  is  to  say,  they  are  much  less  "  prognathous  " 
than  the  apes,  and  the  upper  and  lower  incisors  are  more 
nearly  vertical  in  position,  not  meeting  one  another  at  such 
an  angle  as  in  the  apes.  Mr.  Perrin  (Monthly  Review  Dent. 
Surgery,  1872)  states  that  in  a  gorilla  skull  there  is  an  inch 
of  bone  in  front  of  the  anterior  palatine  foramen  :  in  a  negro 
half  an  inch,  and  in  a  Greek  skull  it  was  close  behind  the 
incisors. 

It  is  generally  said  that  in  man  the  molars  decrease  in 
size  from  before  backwards  ;  that  is  to  say,  that  the  first 
molar  is  the  largest,  while  in  anthropoid  apes  the  contrary 
is  the  case.  Though  this  is  on  the  whole  true,  it  requires 
some  qualification  :  thus  in  certain  lower  races,  such  as  the 
Australian  blacks,  the  second  and  third  molars  are  not 
smaller  than  the  first,  and  of  the  chimpanzee  the  same 
thing  may  be  said. 

There  is  no  diastema  ;  no  sexual  difference  in  dentition  ; 
no  tooth  projecting  beyond  its  fellows,  and  the  teeth  are 
arranged  in  an  unbroken  arch.  The  premaxillary  bones 
become  fused  with  the  superior  maxillary  early  in  life, 
whereas  in  the  Quadrnmaua  they  remain  distinct. 

In  general  terms  it  may  be  said  that  the  dentition  of  th 


416  A    MANUAL    OF  DENTAL    ANATOMY. 


lower  races  of  mankind  differs  from  that  of  the  higher  in  the 
following  particulars  :  the  arch  is  not  so  rounded,  but  is 
squarer  in  front ;  the  teeth  are  larger,  and  are  disposed 
with  greater  regularity  ;  the  wisdom  tooth  has  ample  space 
to  range  with  the  other  teeth,  and  is  a  characteristic  upper 
or  lower  molar,  the  pattern  of  its  grinding  surface  (quadri- 
cuspid  if  it  be  an  upper,  quinquicuspid  if  it  be  a  lower 
tooth)  and  the  disposition  of  its  roots  corresponding  with  the 
first  and  second  molars,  which  do  not  greatly  exceed  it  in 
size.  Specimens  of  negro  skulls  may  be  found  in  which 
there  is  scanty  room  for  the  wisdom  tooth,  and  in  which 
consequently  it  is  a  little  stunted  in  its  development :  on 
the  other  hand,  plenty  of  well  formed  and  well  placed 
wisdom  teeth  may  be  picked  out  of  European  mouths, 
though  as  a  rule  the  wisdom  tooth  is  much  smaller  than 
the  other  molars,  does  not  bear  the  characteristic  pattern 
of  cusps  and  grooves,  has  its  roots  connate,  and  it  is  not 
very  infrequently  a  mere  rudimentary  peg.  The  stunted 
development  of  the  wisdom  tooth  would  seem  to  be  a 
consequence  of  want  of  space  during  its  formative  period ; 
the  upper  wisdom  tooth  is  especially  apt  to  be  cramped  in 
this  way. 

There  is  some  little  evidence  that  tlie  wisdom  tooth  is  in 
process  of  disappearance  from  the  jaws  of  civilized  races  : 
in  anthropoid  apes  the  wisdom  tooth  is  nearly  or  quite  as 
large  as  the  other  molars,  and  shows  no  variability,  whilst 
it  comes  into  place  almost  simultaneously  with  the  canine  : 
in  the  lowest  races  of  mankind  the  wisdom  tooth  appears  to 
vary  but  little,  is  of  large  size,  and  is  seldom  misplaced  ;  in 
highly  civilised  races  it  is  very  variable  in  size,  form,  and  in 
the  date  of  its  appearance,  is  often  misplaced,  and  is  not 
uncommonly  quite  rudimentary.  It  seems  to  be  a  legitimate 
inference  that  a  further  modification  of  the  race  in  the  same 
direction  will  i:esult  in  the  disappearance  of  the  wisdom 
tooth  altogether. 


THE    TEETH    UF   I'RIMATES. 


Some  exception  must  however  be  taken  to  such  general 
statements  :  thus  the  Esquimaux  not  uncommonly  have  the 
wisdom  teeth  small  and  sometimes  crowded  out  of  place  ; 
and  amongst  the  African  races  instances  on  the  one  hand  of 
the  wisdom  teeth  being  small,  and  on  the  other,  of  fourth 
true  moUirs  existing,  are  to  be  met  with. 

Nevertheless,  for  the  present,  a  case  in  which  the  wisdom 
teeth  are  very  small  can  hardly  be  called  a  typical  well-de- 
veloped European  mouth. 

In  many  low  races  (Bosjesman,  Xegro,  Australian,  New 
<'aledonian,  Caffir)  the  second  lower  molar  has  five  cusps, 
just  like  the  first :  this  is  so  in  the  anthropoid  apes,  but  in 
European  races  the  fifth  cusp  is  generally  wanting  in  the 
second  lower  molar. 

It  is  not  a  little  interesting  thus  to  find  that  the  difter- 
ences  which  serve  to  distinguish  the  teeth  of  the  lowest 
.savage  from  those  of  an  European,  are  to  a  certain  extent  the 
.same  with  those  that  mark  the  step  from  a  Quadrumanal  to 
a  human  dentition,  though  of  coxu-se  the  divergence  of  the 
dentition  of  the  savage  from  that  of  the  ape  is  far  greater 
than  is  that  of  the  European  from  the  lowest  savage. 

It  is  very  possible  that  the  larger  development  of  the 
jaws  of  the  savage  may  be  simj)]y  due  to  the  harder  work  to 
which  they  are  put  Avhile  he  is  growing  up.  And  after  the 
attainment  of  adult  proportions,  the  teeth  of  such  a  man 
become  greatly  worn  down  by  reason  of  the  hard  and  often 
gritty  nature  of  his  food. 

It  was  pointed  out  by  Mr.  Mummery,  in  a  very  insti'uctive 
paper  ("Transactions  of  the  Odontological  Society,"  vol.  ii., 
new  series,  1869),  that  destructive  wearing  down  of  the 
teeth  was  of  very  common  occurrence  amongst  rude  (')  races, 
ivhile  the  contrary  is  true  of  highly  civilised  races  ;  tliis  was 

i^)  To  , those  races  mentioued  l>y  Mr.  Mummery  may  be  adiled  the 
mound  builders  of  North  America,  whose  teeth  were  ah^ays  worn  down  to 
au  excessive  extent. 

E  E 


418  A    MANUAL    ni<    DEXTAL    AXATOMT. 

very  likely  due  to  the  admixture  of  earth  aud  other  foreign 
matter  with  the  food.  And,  furthermore,  that  the  occurrence 
of  dental  irrcgiilarities,  due  to  an  insufficient  size  of  the 
arches,  was  comparatively  speaking  unknown  among  the 
ruder  races,  whilst  it  has  been  common  amongst  peoples  of 
more  luxurious  habits  for  a  long  period  of  time. 

The  range  of  variation  in  the  size  of  the  jaws  of  healthy, 
otherwise  well-developed  adults  is  great :  thus  the  smallest 
jaw  (occurring  in  a  man  of  stout  build,  above  middle  height) 
with  which  I  am  acquainted  measures  in  width  only  1|  inch, 
and  in  length  from  back  to  front  1^  inch ;  whilst  the  largest 
(occurring  in  a  gentleman  of  lesser  stature  ;  of  Basque  ex- 
traction, moreover,  which  makes  it  the  more  striking)  (') 
measures  no  less  than  2^  inches  in  width  and  2i  inches  in 
length  :  and  even  larger  dimensions  are  recorded  in  the 
"  Dental  Cosmos  "  of  September,  1876  ;  the  width  being  taken 
between  the  centre  of  the  alveolar  borders  at  the  position  of 
the  wisdom  teeth,  and  the  length  being  measured  on  a  line 
drawn  from  the  incisors  to  another  line  joining  the  two 
wisdom  teeth. 

On  the  whole,  it  must  be  said  that  tliero  are  fewer 
constant  differences  between  the  teeth  of  the  various  races 
of  mankind  than  might  have  been  a  priori  expected  ;  in  fact, 
we  may  almost  say  that  the  teeth  of  savage  man  are  pretty 
much  what  we  should  look  upon  as  an  exceedingly  perfectly 
formed  set  of  teeth  if  we  were  to  see  them  in  the  mouth  of 
an  European. 

(1)  Magitot  (Bullet,  de  la  Soc.  Anthropol.  de  Paris,  1869)  says  :— "  Los 
Basques,  par  exemple,  remarqiiables  par  la  petitesse  extreme  de  leurs 
ilents." 


CHAPTER  XIY. 

THE    TEETH    OF    MARSUPIALIA. 

The  great  sub-class  of  Marsupials,  consisting  of  animals  very 
sharply  marked  off  from  placental  Mammals  by  many  striking 
peculiarities,  and  amongst  others,  by  the  very  helpless  condition  in 
which  the  foetus  is  born,  was  once  very  widely  distributed  over  the 
globe.  Xow.  however.  Marsupials  are  numerous  only  in  Australia, 
where  they  are  almost  the  sole  representatives  of  the  Mammalian 
class  :  there  are  a  few  Marsupials  elsewhere,  as  in  America 
(Opossums)  and  Xew  Guinea :  but  there  are  no  Marsupials  in 
Europe,  most  parts  of  Asia,  and  Africa. 

The  Marsupials  of  America  are  all  Opossums  (I)ulr7j)litd(p'),  and 
this  family  is  not  represented  in  Australia.  There  is  evidence  to 
indicate  that  the  Marsupials  of  Anierica  have  nothing  at  all  to  do 
with  the  Australian  Marsupials,  but  were  derived  from  a  different 
source,  at  the  time  when  Marsupials  abounded  all  over  Eiu'ope. 

The  Marsupials  of  Australia  almost  monopolise  that  country  ; 
thus  Mr.  Wallace  says  of  it  :  '•  The  Australian  region  is  broadly 
distinguished  from  all  the  rest  of  the  globe  by  the  entire  absence 
of  all  the  orders  of  non-aquatic  mammalia  that  abound  in  the  old 
world,  except  two — the  Winged  Bats  (^C/iirnjifcrii),  and  the  equally 
cosmopolite  Rodents.  Of  these  latter,  however,  only  one  family  is 
represented — the  Murida3 — (comprising  the  Rats  and  Mice),  and 
the  Australian  representatives  of  these  are  all  of  small  or  moderate 
size — a  suggestive  fact  in  appreciating  the  true  character  of  the 
Australian  fauna. 

"  In  place  of  the  Quadrumana.  Camivora.  and  Ungulates,  which 
abound  in  endless  variety  in  all  the  other  zoological  regions  under 
equally  favourable  conditions,  Australia  possesses  two  new  orders 
or  sub-classes,  Marsupialia  and  Monotremata,  found  nowhere  else 
in  the  globe,  except  a  single  family  of  the  former  in  America, 

••  The  Marsupials  are  wonderfully  developed  in  Australia,  where 
they  exist  in  the  mo<t  diversified  forms,  adapted  to  different  modes 
of  life.  Some  are  carnivorous,  some  herbivorous,  some  arboreal, 
others  ten-estrial.  There  are  insect-eaters,  root-gnawers,  fruit- 
eaters,  honey-eaters,  leaf  or  grass-feeders, 

E  E  2 


420  A    MAXVJL    OF   DENTAL    ANATOMY. 

"  Some  resemble  wolves,  others  marmots,  weasels,  squirrels,  flying 
squirrels,  dormice,  or  jerboas. 

'•They  are  classed  in  six  distinct  families,  comprising  about  thirty 
genera,  and  subserve  most  of  the  ])urposes  in  the  economy  of  nature 
fulfilled  in  other  parts  of  the  world  by  very  different  groups  ;  yet 
they  all  possess  the  common  peculiarities  of  structure  and  habits 
which  show  that  they  are  members  of  one  stock,  and  have  no  real 
affinity  with  the  old-world  forms,  which  they  often  outwardly 
resemble." — ''  Geographical  Distribution  of  Animals."  p.  ?,'.)\. 

What  Mr.  Wallace  says,  speaking  of  the  creatures  in  their 
entirety,  is  equally  applicable  to  their  teeth. 

In  Australia,  the  present  home  of  the  Marsupials,  repre- 
sentative species  abound;  that  is  to  say,  widely  different 
though  the  animals  really  are,  there  are  mauy  genera  and 
species  which  have  the  habits  of,  and,  as  it  were,  fill  the 
place  of  such  creatures  as  the  Carnivora  and  Insectivora  and 
Eodents  amongst  the  placental  mammalia.  And  not  only 
do  they  possess  something  of  their  habits  and  external  con- 
figuration, but  in  those  characteristic  structures  which  are 
subservient  to  the  creature's  immediate  wants,  the  marsupial 
i-epresentatives  closely  mimic  the  more  highly  organised 
placental  mammals.  Thus  the  teeth  of  an  insect-eating 
marsupial  very  closely  resemble  those  of  a  true  Insectivore, 
though  retaining  certain  eminently  marsupial  characters ; 
in  the  same  way  the  dentition  of  the  marsupial  Thylacine 
mimics  that  of  a  dog  (compare  Figs.  187  and  188). 

And  although  marsupial  dentitions  do  vary  very  much, 
yet  there  are  many  transitional  forms  by  which  we  are  some- 
times able  to  trace  the  successive  modifications  through 
■which  extreme  divergence  has  been  ultimately  attained. 

Just  as  we  ascribe  to  placental  mammals  the  formula — 

.3     1     4      3      ,, 
1  _  c  -  r>  -  ni  -  =  44 
3     K  4      3 

as  the  typical  or  parent  dental  formula,  so  to  the  Marsupials 

we  must  ascribe  the  follov/ing — 

.313      4       ,, 
i_  c-p-m-  =  44. 
3     K  3      4 


THE    TEETH    OF    MAltSUPIALlJ 


That  is  to  say,  tliongh  the  total  number  of  teeth  is  the 
same,  the  marsupial  has  ouly  three  premolars  and  has  four 
true  molars.  The  premolars  (false  molars)  differ  from  the 
true  molars  in  the  greater  simplicity  of  their  crowns,  just  as 
in  most  placental  mammals ;  but,  although,  looking  at  the 
complete  adult  dentitions,  no  hesitation  woiild  be  felt  in 
classing  the  teeth  under  the  heads  of  premolars  and  true 
molars,  yet  there  is  a  curious  anomaly  in  the  succession  of 
the  teeth  which  applies  to  the  whole  of  the  sub-class  Mar- 
supialia,  and  to  some  extent  im-alidates  the  definition  of 
•'premolar"  as  applied  to  their  teeth.  Only  one  of  the 
premolars  (the  hindmost)  has  vertically  displaced  a  milk 
tooth ;  indeed,  the  whole  milk  dentition  of  Marsupials  con- 
sists of  four  milk  molars  (one  on  each  side  of  each  jaw), 
there  being  no  milk  incisors  nor  canines  in  any  known 
marsupial.  It  is  further  pointed  ovit  by  Professor  Flower, 
who  was  the  first  to  fully  describe  these  peculiarities  in  the 
succession  of  marsupial  teeth  ("  Phil.  Trans.,"  1867),  that  the 
extent  to  which  the  solitary  milk  molar  is  developed  varies 
much  in  the  different  families ;  no  trace  of  any  succession 
has  been  observed  in  the  Wombat  ;  m  tlie  Thylacine  (a  dog- 
like creature)  tlie  small  milk  molar  is  calcified,  but  is 
absorbed  or  shed  prior  to  any  other  teeth  being  eruj)ted, 
whilst  in  the  Kangaroos  it  is  retained  till  a  much  later 
period  (see  page  430),  and  in  the  Kangaroo  Rat  (Hyp- 
siprymnus)  the  milk  molar  has  not  yet  given  place  to  its 
successor  at  the  time  when  tlie  last  permanent  molar  has 
come  into  place,  so  that  it  for  a  long  time  ranges  with  the 
other  teeth  and  does  work.  It  is  difficult  to  obtain  veiy 
young  Marsupials,  and  material  for  the  complete  elucidation 
of  the  subject  is  wanting ;  but  I  have  had  the  opportunity 
(jf  making  sections  of  the  jaws  of  several  young  specimens 
( Perameles  and  Halmaturus),  taken  from  the  pouch  by  my 
friend  Prof.   Moseley,  and    I  liave  not  so  far  succeeded  in 


422 


A    MANUAL    OF  DENTAL  ANATOMY. 


Fig.  ISO  ('). 


K 


111111!  Ik 

Wm 


(')  Enamel  and  dentine  of  a  Kangaroo  (Macroiius  major). 
The  dentinal  tubes  in  the  dentine  (A)  are  furnished  with  numerous  short 
branches  at  the  line  of  juncture  with  the  enamel  ;  they  are  dilated,  and  a 
little  bent  out  of  their  course,  while  beyond  the  dilatation  they  pass  on 
through  about  two-thirds  of  the  thickness  of  the  enamel  in  a  straight 
course  and  without  branches.  Only  a  part  of  the  whole  thickness  of  the 
enamel  is  shown  in  the  figure.  1!.  Enamel  penetrated  by  the  tubes. 
C.   Individual  dentinal  tube. 


TUE    TEETH    OF   MARSUPIALIA. 


423 


finding  any  uncalcified  germs  or  other  indications  of  any  of 
the  missing  teeth  of  the  milk  dentition. 

A  furtlier  peculiarity  of  the  Marsnpials  is  the  structure  of 

Fill.  1S7  ('). 


their  enamel,    which   is  penetrated   by  the  dentinal   tubes. 


(')  Upper  and  lower  teeth  of  tlie  Tliylaciiic.  The  rudimentary  milk 
molar,  which  is  absorbed  before  birth,  has  been  placed  over  the  third  or 
last  of  the  premolars,  which  succeeds  to  it  vertically. 

(-)  Upper  and  lower  teeth  of  a  Dog,  which  are  placed  side  by  side  with 
those  of  the  Thylacine  to  show  the  many  points  of  resemblance  between 
the  two  dentitions. 


42i  A    MANUAL    OF  DENTAL    ANATOMY. 


My  father,  some  years  ago,  described  and  figured  the  teeth 
of  a  large  number  of  Marsupial  genera  ("  Philos.  Transac.," 
1850),  and  found  that  although  in  the  different  families  the 
tube  system  of  the  enamel  varied  in  its  richness  and  in  the 
depth  to  -which  the  tubes  penetrated,  yet  it  was  con- 
spicuously present  in  the  whole  class,  with  the  sole  exception 
of  the  Wombats,  in  whom  nothing  of  the  kind  is  to  be  found. 
Prof.  Moselcy's  specimens  have  aftbrded  to  me  the  opportu- 
nity of  studying  the  development  of  this  tubular  enamel,  and 
the  result  of  my  investigations  vvill  be  detailed  elsewhere  : 
but  it  may  be  mentioned  that  the  formation  of  the  enamel 
tube  appears  to  be  precisely  analogous  to  that  of  a  dentine 
tube,  and  at  a  certain  period  the  enamel  cells  have  appended 
to  them  long  processes  like  the  dentinal  fibres.  The  dila- 
tation noticeable  at  the  boundary  line  of  the  enamel  and 
the  dentine  (see  Fig.  186)  is  a  kind  of  clumsy  joint  brought 
about  by  the  coalescence  at  this  point  of  the  tube-forming  cells 
— on  the  one  side  odontoblasts,  on  the  other  enamel  cells. 

There  exists  one  genus  of  flesh-eating  marsupials  whose 
ferocity  is  such  as  to  have  gained  for  them  the  names  of 
wolf  and  tiger,  wdiile  the  resemblance  of  the  head  to  that  of 
a  dog  has  given  origin  to  the  popular  name  of  "  dog-headed 
opossums."  (') 

The  resemblance  to  the  dog  is  in  dentition  even  more 
close  than  in  external  form  :  whilst  retaining  certain  mar- 
supial attributes,  the  teeth  of  the  Thylacine  are,  so  far  as 
their  working  capabilities  go,  almost  exactly  like  those  of  the 
dog.     The  dental  formula  is — 

.  4         1         ■^         4 

1  _     c     -    1)     -    m    _. 

3  1  .3  4 

The  incisors  are   small,  close  set,  and  sharp  edged,  the 


(')  It  lias  of  course  no  real  relationshii)  to  the  true  opossums,  which  are 
not  found  in  Australia. 


THE    TEETH    OF   MAllSUl'IALIA.  425 

outermost  being  somewhat  cuuiniform.  The  canines  are 
stout,  pointed  teeth,  not  quite  so  long  relatively  as  those 
of  the  dog.  The  premolars  are  conical  teeth,  implanted  by 
two  root«,  and  very  similar  to  those  of  the  dog ;  they  are 
followed  in  the  upper  jaw  by  four  molars,  increasing  in  size 
from  the  first  to  the  third,  but  the  last  true  molar  is  again 
a  smaller  tooth. 

The  uj^per  molars  ai-c  all  of  the  "  carnassial "  pattern  ; 
there  is  a  "  blade  "  elevated  into  subsidiary  cusps,  and 
internally  to  this  a  "  tubercle,"  supported  by  a  third  root. 

The  lower  molars  also  bear  some  resemblance  to  the  car- 
nassial teeth  of  the  dog,  consisting  of  a  strong,  sharp-edged 
blade,  with  anterior  and  posterior  subsidiary  ciisps,  the  latter 
being  somewhat  broad  and  tubercular. 

An  allied  animal  (Dasyurus  ursinixs),  though  smaller  than 
the  Thylacine,  and  having  teeth  of  a  less  sectoi'ial  character, 
is  so  destructive  to  sheep,  and  so  fierce  and  untamable,  that 
it  has  earned  the  name  of  "  Tasmanian  Devil." 

AVithin  the  limits  of  the  same  genus,  a  species  (Dasyurus 
viverrinus)  is  to  be  found  in  which  the  molar  teeth  are 
studded  over  with  long  sliarp  cusps,  like  the  teeth  of 
Insectivora,  a  group  which  it  resembles  both  in  its  habits 
and  food. 

A  number  of  smaller  ^larsupials  approximate  in  their 
dentition  more  or  less  to  the  Insectivorous  type,  whilst  a 
tolerably  complete  chain  of  existing  forms  serves  to  bridge 
over  the  gap  between  the  rapacious  Dasyurida;  and  the 
herbivorous  Kangaroos  and  Wombats. 

Amongst  the  Opossums  the  larger  species  have  large 
canines,  and  a  dentition  iu  its  general  features  approxi- 
mating to  the  Dasyuridie ;  they  feed  upon  birds  and  small 
mammals,  as  well  as  upon  reptiles  and  insects,  while  the 
smaller  species  are  more  purely  insectivorous. 

Myx-mecobius,  a  small  Australian  Marsupial  of  insectivo- 
rous habits  and  dentition,  is  remarkable  as  having  teeth  in 


426  A    MANUAL    OF  DENTAL    ANATOMY. 

excess  of  the  number  of  tlie  typicul  mamraaliau  dentition, 
having 

.  4         1  :'.         G 

1   _     C     -     p     _    111    _. 

In  the  Phalangers,  nocturnal  arboreal  animals  found  in 
Australia  and  a  part  of  the  ^lalay  Archipelago,  tlic  canines, 
though  present,  are  feeble  ;  an  interspace  also  sei)arates  the 
incisors  from  the  molar  series. 

The  lower  incisors,  reduced  to  a  single  pair,  are  procum- 
bent, and  grow  from  persistent  pulps  ;  and  a  slight  exagge- 
ration of  the  peculiarities  of  the  dentition  of  the  Phalangers 
brings  us  to  that  of  the  Kangai'oo  Kats. 

The  name  "  Kangaroo  Rats"  (Hypsiprymnus)  is  applied 
to  a  genus  consisting  of  about  a  dozen  species  ;  they  are  all 
small  creatures,  not  much  larger  than  rabbits,  but  having 
the  general  proportions  of  kangaroos.  They  are  quiet, 
gentle  little  creatures,  of  strictly  herbivorous  habits,  and 
they  are  interesting  to  the  odontologist  as  possessing  a 
dentition  which  throws  some  light  upon  several  anomalous 
extinct  forms,  whose  habits  and  affinities  have  been  the 
subject  of  much  controversy. 

The  dental  formula  is 

.3  1  1  4 

1  0^1  4 

The  first  })air  of  upper  incisors  are  sharply  })ointed,  are 
directed  nearly  vertically  downwards,  and  grow  from  per- 
sistent pulps.  The  second  and  third  do  not  grow  from 
persistent  pulps,  and  their  worn  crowns  do  not  attain  to  the 
same  level  as  those  of  the  first  pair. 

All  three  pairs  are  antagonised  by  the  single  pair  of  large 
procumbent  lower  incisors,  of  -which  the  sharp  points  meet 
the  first  pair  of  upper  incisors,  while  the  oldiquely-worn 
surface  behind  the  cutting  edges  impinges  against  the  second 
and  third  upper  incisors. 

The  arrangement  of  the  incisors,   and  the  sharpness  of 


THE    TEETH    OF   MARSUPIALIA. 


427 


the'r  cutting  edges,  ayg  calculated  to  effect  the  same  objects 
as  those  attained  by  the  iucisors  of  a  rodent ;  a  still  closer 
resemblance  would  be  brought  about  by  the  dwindling 
(which  occurs  in  other  genera)  and  final  disappearance  of 
the  second  and  third  upper  incisor.s,  and  a  compensating 
extra  development  of  the  first  pair. 

The  canines  are  not  large  ;  yet  they  are  not  so  small  as 


s\al  Sf=e^ 


to  be  termed  rudimentary  ;  in  the  lower  jaw  they  are 
absent. 

Only  one  premolar  exists  in  the  adult,  and  this  is  a  very 
peculiar  tooth  ;  its  crown  is  very  long  from  back  to  front 
(at  least  twice  as  long  as  any  of  the  molars,  and  in  some 
species  as  long  as  three  of  the  molars),  and  consists  of  a 
finely  furrowed  blade  with  a  sharp  edge ;  the  blades  of  the 
upper  and  lower  teeth  slide  over  one  another.  Behind  this 
there  are  four  true  molars,  with  square  quadricuspid  crowns, 
which  become  much  worn  down  by  use. 

The  third  premolar,   the   tooth  to    which    attention  has 


(')  Upper  and  lower  teeth  of  Hypsiprymnus  (Bettongia)  (Graii  ?).  Tbe 
flentition  represented  is  that  of  the  adult  animal,  the  permanent  premolar 
{pm  3)  being  already  in  place. 


423  A    MANUAL    OF   DENTAL    ANATOMY. 

already  been  drawn  on  account  of  its  size  and  other  pecu- 
liarities, by  virtue  of  its  great  size  displaces  not  only  the 
milk  molar,  to  which  it  is  the  legitimate  successor,  but  also 
turns  out  the  second  premolar,  a  tooth  belonging  to  the 
"  permanent "  series. 

In  this  particular  the  succession  of  the  teeth  in  the 
Hyj)siprymnus  is  the  same  as  that  of  the  true  kangaroos, 
which  may  be  understood  by  a  reference  to  fig.  190. 

There  are  two  extinct  Marsupials,  known  only  by  their 
jaws,  which  have  been  the  subject  of  much  controversy. 
Professor  Owen,  basing  his  arguments  largely  upon  the 
presence  of  premolars  which  possessed  elongated  and  sharp- 
edged  blades,  held  that  Plagiaulax  and  Thylacoleo  were 
carnivorous,  saying  of  the  latter  that  it  possessed  the  simplest 
and  most  effective  dental  machinery  for  predatory  life  known 
among  Mammalia  ;  Dr.  Falconer,  in  the  case  of  the  first,  and 
Professor  Flower  in  the  case  of  the  Thylacoleo,  having  shown 
this  view  to  be  untenable,  or  at  least  unsupported  by  adequate 
evidence. 

The  clue  to  the  nature  of  the  great  blade-shaped  teeth  of 
these  two  extinct  creatures  is  afforded  by  the  form  of  the 
premolar  of  the  herbivorous  HyjDsiprymnus  (see  fig.  189). 
The  incisors  were  reduced  in  numl^er  and  were  large  ;  the 
teeth  between  them  and  the  large  premolar  were  stunted  ; 
but  both  these  points  are  true  of  the  herbivorous  kangaroos. 
The  Thylacoleo  differs,  however,  from  all  known  animals  by 
the  immense  size  of  the  thin-edged  premolar  (worn  fiat  in 
aged  animals),  and  by  the  rudimentary  condition  of  its  true 
molars.  But  its  incisors,  lying  forwards  and  closely  ap- 
proximated in  the  middle  line,  are  particularly  imsuitable 
for  catching  and  holding  anything  alive  and  struggling, 
whilst  the  nearest  resemblance  to  the  blade-shaped  tooth  is 
to  be  found  in  harmless  herbivorous  creatures,  so  that  the 
balance  of  evidence  is  much  against  Professor  Owen's  vicAv. 
It  has  been  cited  here  merely  as  an  instance  of  how  the 
evidence  afforded  by  teeth  alone  may  bo  misleading. 


THE    TEETH    OF   JUESUFIALIJ. 


429 


The  Kangaroos,  comprising  many  species  of  very  varying 
size,  are  all  docile  creatures  ( with  the  exception  of  a  few  old 
males),   of  herbivorous  habits ;    they    in    some   particulars 
recall  the  ruminants. 
Their  dental  formula  is 

()  1  4 

0^1  4 

The  three  pairs  of  upper  incisors  are  more  equal  in  size 


.  3 

1  - 
1 


Fig.  190  (' 


A'irl  Sizf^ 


than  in  the  Hypsiprymnus,  and  the  central  pair  do  not 
grow  from  persistent  pulps.  The  lower  incisors  are  very 
peculiar  teeth :  they  grow  from  persistent  pulps,  are  pro- 
€ambent,  projecting  forwards  almost  horizontally,  and  arc 
very  much  flattened  from  side  to  side,  their  outer  surfaces 
being  but  slightly  convex,  and  their  inner  surfiices  flat,  with 
a  median  ridge.  Their  margins  are  almost  sharp.  There  is 
an  unusual  amoimt  of  moliilitv  between  the  two  halves  of  the 


(')  Upper  and  lower  teeth  of  Hahnaturns  ualabatus.  The  penuancnt 
premolar  is  not  yet  ernxjted,  and  is  sho\vn  in  its  crypt  :  when  it  comes 
into  its  place  it  will  displace  the  milk  molar,  and  one  of  the  anterior  pre- 
molars as  well.  In  the  upper  jaw  a  rudimentary  canine  is  shown.  The 
point  of  the  lower  incisor  wouhl  lit,  in  closure  of  the  mouth,  behind  the 
long  anterior  upper  incisor,  but  the  width  of  the  page  did  not  admit  of  the 
teeth  being  placed  in  their  true  relative   positions  without  reduction   in 


430  A    MAXVAL    OF   DENTAL    ANATOMY. 

lower  jaw,  so  that  these  two  teeth  can  be  to  a  slight  extent 
separated  from  one  another. 

The  upper  canine  is  often  present  as  a  very  minute 
rudiment,  but  in  no  kangaroo  does  it  attain  to  a  greater 
size. 

The  dentition  of  the  Kangaroo  is  somewhat  pei-plexing 
to  the  student,  for  two  reasons  :  the  one,  that  the  L\st  or 
third  permanent  j)remolar  not  only  displaces  the  solitary 
milk  molar,  but  also,  as  in  Hypsiprymnus,  on  account  of  its 
gi'cater  size,  the  second  permanent  premolar,  which  was  in 
front  of  the  milk  molar  ;  and,  besides  this,  in  animals  past 
adult  age,  teeth  are  shed  off  from  the  front  of  the  molar 
series  till  at  last  only  the  last  two  true  molars  on  each  side 
remain. 

Thus  the  dentition  of  the  kangaroo  at  successive  ages 
may  be  thus  represented  : 

.3         0  111  4 

1    _-     c     -    p     -    dm     -    ni    _, 

1  0^1  1  4 

or,  in  all,  six  molar  teeth.  Then  the  third  premolar  dis- 
places the  second  true  permanent  premolar  as  well  as  the 
milk  molar,  and  we  have 

.301,  ,4 

1   _    c     -    p    -    (a  new  one)    m  -, 

or,  in  all,  only  five  molar  teeth. 

Then,  one  after  another,  teeth  are  shed  off  from  the  front 
of  the  molar  series,  just  as  in  the  Phacocluei-us  (see  page 
328),  till  all  that  is  left  is 

3         0         0  2 

^   T   W.  1^    0   ^^    2- 

The  milk  molar  of  the  kangaroo  is  a  fully-developed 
tooth,  which  takes  its  place  Avith  the  other  teeth,  and  is  not 
distinguished  from  them  by  any  special  characters,  so  that 
mere  inspection  of  the  jaw  of  a  young  Kangai'oo  having 
it  in  place,  at  the  same  time  with  a  premolar  in  front  of  it 


THE    TEETH    OF   MARSUl'IALIA. 


aud  four  true  molars  behind  it.  would  not  lead  au  observer 
to  suspect  its  real  nature. 

No  existing  creature  serves  to  connect  tlie  Kangaroos 
closely  with  the  wombat,  but  the  extinct  Diprotodon  appears 
to  have  iu  a  measure  bridged  across  the  gap. 

The  Wombats  (Phascolomys)  are  heavily-built,  inoffensive 
creatures,  which  burrow  iu  the  ground  aud  subsist  largely 
upon  roots.     In  their  dentition  they  closely  simulate  the 


Frn.  IPl  ('). 


-fXaJ.Si'ze. 


Jim 


C^ 


Rodents,  as  they  possess  but  a  single  pair  of  chisel-edged 
iucisors  in  either  jaw,  growing  from  persistent  pulps,  and 
embedded  in  veiy  deep  and  cun^ed  sockets.  These  differ  from 
the  corresponding  "  dentes  scalpi-arii "  of  true  Rodents  in  that 
there  is  a  complete  investment  of  cement,  which  passes  over 
the  enamel  in  front  of  the  tooth  as  Avell  as  covering  its  back 
and  sides.  They  are  unlike  the  teeth  of  other  Marsupials 
in  their  structure,  as  the  dentinal  tubes  do  not  penetrate 
the  enamel,  which  is  therefore,  probably,  harder  and  denser 
and  so  less  readily  worn  away. 

The  molar  teeth    also   grow  from  persistent  pulps,  and 


432  A    MANUAL    OF   DENTAL    ANATOMY. 


fire  very  deeply  grooved  upon  tlieir   sides,  so    that    their 

grinding  surfaces  are  uneven. 

Their  dental  formula  is, 

10]  4 

1  J    c-    p   3   m   -^. 

The  first  tooth  of  the  molar  series  is  a  single  column, 
whereas  the  deep  grooving  of  the  others  divides  them  into 
two  columns,  so  that  its  simpler  appearance,  as  well  as 
analogy,  would  indicate  that  it  is  a  premolar.  But  no 
succession  whatever  has  been  ol)Scrved  in  the  wombats. 

The  adai)tive  resemblance  to  the  dentition  of  the  true 
Rodents  is  exceedingly  close,  though  the  Wombat  is  an 
undoubted  Marsupial ;  and  the  very  closeness  of  the  imita- 
tion is  an  exemplification  of  the  fact  that  adaptive  charac- 
ters are  very  apt  to  mislead,  if  used  for  the  purposes  of 
classification. 

Extinct  wombats,  of  very  much  larger  size  than  the 
recent  species,  are  found  in  the  later  tertiary  dei)OsitH  of 
Australia. 

Amongst  the  Marsupials  there  is  a  pretty  little  arboreal 
creature  (^Tarsipes),  not  larger  than  a  small  rat,  which  sub- 
sists upon  insects  and  the  nectar  of  flowers,  which  it  reaches 
by  means  of  a  long  protrusible  tongue.  Its  molar  teeth 
are  rudimentary,  variable  in  number,  and  are  soon  shed ; 
the  lower  incisors,  which  are  procumbent,  are  however  re- 
tained, as  are  a  few  small  teeth  which  are  opposed  to  them 
above. 

The  wonderful  diversity  of  the  forms  into  which  the 
Marsupials  have  branched  out  in  Australia  seems  to  prove 
that  they  have  been  established  in  that  region,  and  have 
been  without  the  competition  of  more  highly  organised 
placental  Mammals,  for  a  prodigious  length  of  time  ;  and 
one  cannot  better  conclude  the  very  brief  survey  of  the 
teeth  of  Mammalia  which  hasl)ecn  attempted  in  this  volume 

(')  Uppei-  and  knver  teeth  of  'Wombat  (riiasculomys  wombat). 


THE    TEETH    OF  MARSUPIALIA. 


than  by  calling  the  reader's  attention  again  to  the  character  of 
the  Mai'supial  fauna  :  this  microcosm,  in  which  every  place 
is  filled  by  a  Marsupial  that  mimics  the  placental  Mammal 
which  it  represents,  for  nowhere  can  we  more  plainly  see 
the  workings  of  natural  selection  than  in  areas  thus  isolated 
and  deprived  of  immigrant  creatures  for  countless  ages. 

In  the  foregoing  pages  much  stress  has  been  laid  upon 
the  variability  of  animals,  and  the  agencies  by  means  of 
which  the  variations  have  been  preserved  and  intensified, 
so  to  speak,  so  that  ultimately  permanent  hereditary  modi- 
fications have  been  the  result ;  and  it  is  possible  that  in 
laying  this  aspect  of  the  matter  prominently  before  the 
reader  an  impression  of  too  great  instabilit}-  may  have  been 
conveyed ;  and  thus  the  forms  of  creatures  made  to  appear 
more  plastic  and  more  shifting  than  they  really  are,  for  it 
is  hardly  possible  to  realize  the  enormous  lengths  of  time 
during  which  the  agencies  have  been  at  work,  and  without 
which  they  would  have  been  powerless  to  produce  profound 
alterations. 

The  process  which  we  term  inheritance  is  constantly 
reproducing  animals  which  are  minute  copies  of  their 
parents  ;  copies  which  are  even  more  exact  than  we  can  at 
first  sight  realise. 

Thus,  even  amongst  different  species  of  the  same  genus, 
whose  teeth  are  apparently  quite  similar  so  far  as  their 
number  and  pattern  goes,  differences  exist  which  are  con- 
stant for  the  species,  and  which  may  be  brought  into 
prominence  by  any  method  of  investigation  wliich  is 
sufficiently  accurate. 

A  plan  of  representing  in  the  f>rm  of  diagrams  certain  of 
the  characteristics  of  an  animal's  dentition,  by  means  of 
which  difTerences  of  proportion  so  slight  as  to  be  barely 
recognisable  by  an  inspection  of  the  teeth  are  brought  con- 
spicuously into  view,  has  been  devised  by  Mr.  Busk  ("Proc. 
Roy.  Soc.  1870  ")■ 


434  A    MANUAL    OF   DENTAL    ANATOMY. 

If  "  odontogmms  "  of  various  Felidic  be  constructed,  differ- 
ences between  them  will  be  apparent  at  a  glance,  although 
the  forms  of  the  several  teeth  in  this  family  are  so  very 
closely  similar,  that  nothing  but  the  veiy  closest  observa- 
tion would  have  detected  the  smallest  diflerence  between 
tliem. 

I  have  not  practically  tested  the  applicability  of  this 
method  of  comparison,  but  it  is  said  that  these  diagrams, 
embodying  as  they  do  only  one  set  of  facts  about  a  dentition, 
have  proved  less  useful  than  might  have  been  anticipated, 
and  occasionally  may  even  prove  misleading. 


INDEX. 


A. 


Absorption  of  teeth 

-Verodus,  teetli  of     .         .         .     . 

Adaptive   moditieatiou,   meiuiiii^^ 

of  term 

Alveolar  proeesses   .         .         .     . 
„        process,  development  uf 
Alveoli,  attachment  by  means  of 
Alveolo-dental  membrane        110, 
Anarrhicas,  teeth  of    . 
-Vuchylosis  of  teeth 
-Vngler,  teeth  of  . 
Anoplotherium,  teeth  of.        .     . 
Anteatei-s,  teeth  of       . 
-Vnthropoid  apes,  teeth  of 

„  „    order    of    erup- 

tion of  teeth  in 
,,  „     comparison      of 

with  man 

Antrum 

„    teetli  ill  relation  with 
Archreopteryx,  teeth  of  .         .     . 
^Vi-maiUllo,  teeth  of      . 
Ai-ticulation  of  the  lower  jaw 
„       ■        „  teeth  with  one 

another 
.b-ticular    process,    share    of    in 

growth  of  jaw  .         .         . 
Artiodactyle  unsulat;i     .         .     . 
-Vttachment  of  teeth     . 

„  „   by  membrane 

,,  „    by  hinges 

.,  „   by  anchylosis 

„  ,,   bv  sockets 

Aye-aye,  teeth  ^f.     "   .         .271, 


PAr.B 
Barracuda  pike,  teeth  of  .  .  235 
Basal  ridge,  orcingulum  .     .     10 

Basement    membrane    of   tooth- 


Babiinssa,  teeth  of 
Baboon,  teeth  of 
Bala;nidai,  whalebone  of 


germ         .... 

.  171 

267 

Bats,  teeth  of 

.  401 

24 

„     milk  teeth  of 

.  402 

192 

Bear,  teeth  of 

.  387 

ib. 

Beaver,  teeth  of 

45,  372 

212 

Betton-ia,  teeth  of 

.  427 

229 

Birusjiids,  Iniman 

.     14 

207 

l{il(i])li<idont,  meanhig  of  tcnu 

.  317 

228 

Birds,  teetli  of 

.  260 

;i32 

Boll,  on  dentinal  fibrils 

66-68 

30.5 

Bone,  primary 

.   168 

409 

„       development  of . 

.    ib. 

Brown  striie  of  lletzins 

.     52 

ib. 

Buccinator,  attachment  of   . 

.     30 

Bunodont,  meaning  of  term 

.  328 

410 
27 

29 
261 

('. 

305 

Calcified  teeth 

.      3 

32 

Calcification,  dates  of  in  the  st 

ve- 

ral  teeth . 

.   146 

5 

„          process  of 

.  148 

„           of  enamel    . 

.   151 

188 

,,           of  dentine 

.  158 

325 

,,          of  vaso-dentine 

.   164 

202 

,,           ofosteo-     „ 

.   165 

ib. 

,,          ofcementum 

.   166 

203 
207 

Calcoglobulin       . 

.  150 

Calcosphcrites 

.  149 

212 

Camel,  teeth  of    . 

.  337 

404 

Canaliculi  of  cementum  . 

.     97 

Canidie,  teeth  of  . 

.  379 

Canine  teeth  of  ruminants 

.  282 

,,          of  lemurs 

.  283 

„          oforeodon  . 

.    ib. 

329 

„          of  mole    . 

.  284 

408 

„          of  insectivora 

.    ib. 

312 

Canine,  definition  of   . 

.  280 

436 


INDEX. 


t'liuiuc,  sexual  devflopiiient  of     . 

I'AdE 

Lecus-satiou  of  enamel  prisms .     . 

■AliE 

50 

.,       true  signification  of  term 

281 

Deer,  teeth  of      . 

335 

Canines,  human       .         .         .     . 

12 

Ueficien 

•ies  of  teeth  in  hairy  men 

276 

Capybaru,  teeth  of       .         .     ;:G8-370 

„          in     Turkish 

,,         nioliir  teeth  of 

296 

dogs.     . 

275 

<  'archarias,  teeth  of     . 

219 

Dentine 

calcification  of 

158 

Caniassial  tooth       .         .         .     . 

376 

j^ 

composition  of  .         .     . 
fibrils  of        . 

68 

Carnivora,  milk  dentition  of 

;i77 

j' 

64 

,,          teeth  of      .         ... 

374 

., 

genu         .         .         113, 

132 

Caraivorous    dentition,    general 

\ 

globular 

163 

iharacter  of 

376 

,, 

granular  layer  of      .     . 

72 

Cement  oigan       .... 

144 

^ 

interglobular  spaces  of . 

ib. 

,,            doubtful    existence 
of 

145 

;; 

matiix  of         ... 
modifications   of  in  la- 

58 

Cement,  over  crowTi  ol'  tooth 

100 

byrinthodon 

79 

Cemeutuni 

95 

,, 

,,        in  lepidosteus 

78 

„        rudimentary   .         .     . 

96 

in  manatee  83,  347 

,,        structure  of 

ih. 

„         in     megathe- 

distribution of         .     . 

42 

rium. 

84 

,,         calcification  of     . 

166 

^ 

„         in  myliobates 

81 

Ceratodus,  teeth  of.         .          .     . 

238 

1' 

,,         in  varanus    . 

76 

IJcstracion,  teeth  of     . 

221 

osteo-    .... 

88 

Cetacea,  teeth  of     . 

;;07 

^, 

plici-        .... 

76 

tJhsetodonts,  teeth  of   . 

235 

,, 

secondary     . 

93 

Cheirom^■s,  niillc  teeth  of        .     . 

406 

sensitiveness  of        .     . 

71 

„    '      teeth  of     , 

271 

sheaths  of  Neumaim  in 

63 

Chelonia,  teeth  of           ... 

241 

lei-mination  of  tubes  of 

72 

Chiroptera,  teeth  of     .         . 

101 

'. 

theories    as    to    forma- 

Cingulum, detinitiou  of           .     . 

11 

tion  of  .         .         .     . 

162 

,,           developed  into  addi- 

lubes    .... 

61 

tional  cusps  . 

292 

J, 

unvascular        .         .     . 

56 

Coatimuudi,  teeth  of       . 

;i86 

,, 

varieties  of   . 

91 

Cobra,  teeth  of    .... 

247 

,, 

vascular  .        .        .     . 

82 

Complex  teeth,  manner  of  forma- 

,, 

•\-aso-     .... 

ib. 

tion  of     .         .         .          288  et 

seq. 

Dents  en  vcloui-s,  en   brosse,  en 

Contour  lines  of  Owen 

61 

cardcs 

235 

Coronoid  process,  use  of  in  growth  189 

Dermal 

■spines  of  fish  .         .       2, 

219 

Correlations  of  growth    .         ,     . 
Crocodiles,  teeth  of     .         .         . 

275 

Desmodus,  teeth  of         ... 

401 

256 

Development  of  the  teeth     113  et 

seq. 

„          implantation  of  teeth 

^, 

commencement     of 

of       .         .         .     . 

213 

113 

128 

succession  of  teeth  in 

258 

^^ 

in  eel         .        .     . 

121 

Crypts  of  developing  teeth  . 

180 

'^ 

in  fishes 

115 

Curvatures  of  dentinal  tubes 

60 

'.' 

in  lizards  .         .     . 

125 

Cusps,  formation  of         .         .     . 

292 

in  mammalia  . 

128 

Cuticula  dentis    .... 

99 

,, 

in  reptiles .         .     . 

122 

Cynodraco 

259 

'' 

in  snakes 

126 

Cystophora,  teeth  of    . 

o91 

of  the  true  molars  . 

138 

Czermak,  interglobular  spaces  of 

74 

Diastem 

of  the  jaws         .     . 
of  the  alveolar  pro- 
cesses     .         .     . 

176 

198 
413 

1). 

Dicynod 

on,  teeth  of         .        .     . 

243 

Dinoceras,  teeth  of      .         . 

341 

Dinotherium,  teeth  of     .         ."49, 

360 

Dasyurus,  teeth  of  .         .         .     . 

425 

Diodon, 

teeth  of      .         .         .     . 

231 

Deciduous  dentition     . 

297 

Diprotoc 

on,  teeth  of    .        .        . 

431 

437 


PAOE 

.  378 
.  ib. 
2,  217 
.  343 
.  345 
.    ih. 


Dog,  teeth  of 
,,     variation  in  toutli  of 
Dog-fish,  teeth  of     . 
Dryptodon,  teeth  of     . 
Dugong,  teeth  of    . 
,,         tusks  of  male 


Edentata,  teeth  of        .         .         .  304 

Eel,  development  of  teeth  of        .  121 

„     enamel-tipped  teeth  of    .  40,  210 

Elasmobranih  ti?h,  teeth  of     11-5,  215 

Elephant,  milk  teeth  of       .         .  354 

,,         molars  of         .         356,  361 

,,         sueeessiou  of  teeth  iu  .  354 

tusks  of        .         .         .350 

Enamel  .         .         .         .       41  ct  scy. 

,,         al)sence  of      .  .41 

,,        eavities  in  ...     52 

„        eakitication  of        .         .   151 

cells 153 

„  „     calci6cation  of        .    ib. 

„        cutrj-  of  dentinal  tubes 

into  .  .  .  .  53 
„  fracture  of  .  .  .44 
„        germ  .         ...  114 

„  ofsargus  .  .  .55 
„  of  marsupials  .  .  .  53 
„  organ,  development  of  .  134 
„  ,,   external  epithelium 

of    .         .         132,  153 
„  „    internal  epitheliuiii 

of         .         .    132,  153 
„            ,,    neck  of      .         .     .  132 
,,        pigment  iii     .         .         .     52 
„        prisms  of            .         .     .     44 
„        rudimentary                    .41 
„         organ,     stellate     reticu- 
lum of    .         .     .  135 
,,            „         striation  of        .     50 
„        theories  of  formation  of .  154 
,,        tubular           .         .         .     53 
Eruption  of  teeth,  date  of        .     .  193 
„            „          mechanism    of  190 
External  pterygoid  muscle,  action 
of 34 


Felidw,  teeth  of      .        .        .  .  375 

Fibrils  of  dentine  .  .64 

Fishes,  teeth  of       .        .        .  .  214 

,,        classification  of       .  .    ib. 

„        structure  of  teeth  of  .  .  236 

Foetus  (nine  mouths),  teeth  of  .  179 


PAGE 

Follicle,  dental    .        .        .133 

,  142 

Frog,  teeth  of 

239 

G. 

Galeopithecus,  teeth  of 

401 

Genu,  tooth     .... 

113 

Glenoid  cavity,  form  of  iu  carni- 

vora 

34 

,,            ..        form  of  in  herbi 

vura    . 

.    i't. 

Globular  dentine 

163 

Goodsir,  special  views  of 

129 

Grampus,  teeth  of 

308 

Granular  layer  of  dentine 

72 

Growth  of  the  jaws 

177 

Gubei-naculum 

145 

Gum,  the     .... 

108 

Gymnodouts,  teeth  of 

230 

II. 

Haddock,  teeth  of       .        . 

.  211 

Hair  and   teeth,   correlation  be 

tween           .... 

.  275 

Hairy  men  .... 

Hairless  dogs,  teeth  of    . 
Hake,  dentine  of 

.  276 

.  275 

.     87 

„      hinged  teeth  of    .        20C 

,228 

Halicore          .... 

.  345 

Halmaturus,  teeth  of 

.  429 

Hare,  teeth  of          .         .         . 

.  367 

Hatteria,  teeth  of         .         .    239,  243 

Hedgehog,  teeth  of 

.  394 

Helodermis,  teeth  of    . 

.  242 

Hesperornis,  teeth  of      . 

.  263 

,  Hinged  teeth 

.  202 

Hipparion,  teeth  of 

.  321 

Hippopotamus,  teeth  of 
Homalodontotherium 

.  331 

.  2S(; 

Homologies  of  the  teeth 

.  278 

:  Horny  teeth    .... 

.       3 

Horse,  teeth  of    . 

.  321 

Human  teeth,  forms  of    . 

.  414 

Huxley,  Prof.,   special  views  on 

development     .         .         .15 

2,  157 

Hya>na,  teeth  of      .         .         . 

.  381 

Hva;nodon,  teeth  of    . 

.  385 

Hydrophis,  teeth  of 

.  247 

Hydromys,  teeth  of     . 

.   366 

Hypsiprymnus,  teeth  of  . 

.  427 

Hyrax,  teeth  of 

!                I. 

Ichthyosaurus 

.  348 

.  258 

Ichthyornis,  teeth  of    . 

.  261 

433 


Ig;uanodon,  teeth  of 
Incisors,  definition  of  . 

,,         liuniiin,  description  of 
Insectivora,  teeth  of    . 

,,  characteristic  niolai 

of 
Internal  pterygoid  muscle 
Interglobular  spaces    . 


Kangaroos,  teeth  of 
Kangaroo,  enamel  of 


L. 


403 

•283 

237 

7G 

74 

60 

241 

228 

•53 


I.alnrintliodou,  teeth  of 

,,  dentine  of 

Lacuna!,  of  cemcntum     . 

„        development  of 

„         encapsuled 

in  pits  of  enamel 

,,        of  Howship 
Lanma,  teeth  of  . 

„        dentine  of . 
Lemurs,  teeth  of 

,,         canines  of 
Lepidosiren,  teeth  of    . 
Lepidosteus,  dentine  of   . 
Leptothrix,       in      interglohu 

spaces 
Lines  of  Schreger    . 
Lizards,  teeth  of 
Lophius,  teeth  of    . 
Lumen,  appearance  of 


M. 


ZSlachairodus,  teeth  of     .         .     .  384 
;Mammalia,  teeth  of     .         260  ct  fcq. 
„  tj'pical  dentition  of  .  279 

^Mannuoth,  tusks  of 
Man,  teeth  of  . 

„     teeth  of  difierent  races 
Manatee,  teeth  of 
„         enamel  of 
,,         dentine  of     . 
^Mandible 

Mark  of  horses'  incisors 
Marsupiatia,  teeth  of 

,,  milk  teeth  of 

,,  peculiar  enam 

iUasseter  muscle  . 
Mastication,  muscles  of  . 


IXDEX. 

PACE 

|'a<;e 

.  1'43 

Mastodon,  teeth  of 

.  357 

.  280 

„          molars  of 

.  360 

;» 

milk  teeth  of      . 

.  359 

.  394 

Maxilbe,  description  of  . 

.    24 

s 

„         development  and  growth 

.  400 

of       .         .         . 

.  176 

.     34 

„         Y-shaped . 

.  201 

2,  74 

Meckel's  cartilage 

.  177 

Megatherium,  dentine  of 

.     84 

Membrana  eboris     . 

158,  140 

„         preformativa 

.  171 

Mental  foramen,  position  of 
Milk  dentition,  character  of 

.  183 

.  430 

.  297 

.    53 

„              rudimentarj' 

.  301 

Molars,  definition  of    . 

18,  280 

Mole,  teeth  of 

.  397 

„      milk  teeth  of      . 

.  3f<9 

Monkeys,  teeth  of  . 

.  407 

.  240 

Monodon,  teeth  of 

.  309 

.     80 

Monotiemata,  teeth  of    . 

.  304 

.     96 

Muscles  of  mastication 

.     33 

.   168 

iliislc  deer,  teeth  of 

.  336 

.  169 

Mustelidic,  teeth  of      . 

.  385 

.     54 

Myliobates,  dentine  of    . 

.     82 

.   197 

teeth  of     . 

.  223 

.  216 

Mynnecobius,  teeth  of    . 

.  425 

.     90 

Myxiiie,  teeth  of 

.  215 

350 

.     .       1 

of    .  415 

.  347 

.     .     45 

83,  347 

.     .     30 

.  289 

.     .  419 

.  421 

I  of  53,423 

.     34 

.     .     33 


X. 


Narwal 309 

,,        teeth  of  .         .         .         .    ib. 

Nasmyth's  membrane,  nature  of  .     99 

Neck  of  enamel  organ  .         .         .132 

„    of  tooth  .        .        .     .      7 

Nerves  of  dentine         .         .         .71 

„       of  the  pulp    .         .  71,  106 

„       of  the  teeth       ...     37 

Neumann,  sheaths  of      .        .     .    63 

Newt,  teeth  of     .         .         .         .240 


Oblique  ridge  of  human  molars    .     19 
Odontoblast  cells  .         .     69,  158 

Odontoptervx,  teeth  ot     .         .     .  260 
Odontorniths,  teeth  of  .         .  261 

Ophidia,  development  of  teeth  of  125 
Opossum,  teeth  of  .  .  .  425 
Orang,  teeth  of  .  .  .  .  410 
Oreodon,  teeth  of  .  .  .  332 
,,  canines  of  .  .  .  ib. 
Oryeteropus,  dentine  of  .  .82 
Osseous  fish,  teeth  of  .  .  .  224 
Osteoblasts  .         .         .         .166 


INDEX. 


439 


Osteoclasts 197 

Osteodentine        .         .         .         .88 
,,  in  tt'cth  of  rodents    .  371 

,,  in    teeth    of    si)erni 

whales  .      92,  309 

Ostracion,  dentine  of       .         .     .     86 

Otaria,  erosion  of  teeth  of    .        .  389 

„       teeth  of       .        .        .     .    ib. 


Papilla,  formative  . 

PaiTot  fish,  teeth  of     . 

Periosteum,  alveolo-dental 

I'erissodactj-le  iiuy;ulata,  teeth 

Permanent  teeth,  eruption  of 
,,  „       development 

Persistent  dental  capsule     . 

Phacochierus,  teetli  of     . 

Phalauger,  teeth  of 

Pharyngeal  teeth     . 

„  ,,      of  carp     . 

„  „  pike 

,,  ,,  racliiodon 

,,  „  scarus  . 

rhofid;e,  teeth  of    . 

Pigment  in  enamel 

Pig,  teeth  of  ... 

Pike,  teeth  of       .         .         .   '1 

Plagiostomi,  teeth  of 

Plagiaulas,  teeth  of     . 

Plicidentine     .         .         .         . 

Poison  fang,  development  of 
„  mechanism  of 

,,  structure  of 

„  succession  of 

Poison  gland 

Porcupine,  enamel  of 

Premolai-s,  human 

,,         detir.ition  of   . 

Primates,  teeth  of 

Pristis,  teeth  of        .         ;         . 

Proboscidea,  teeth  of  . 

„  affinities    with     i 

dents   . 

Procvonida?,  teeth  of 

Proteles,  teeth  of 

Pseudoscarus  .... 

Pterodactyls,  teeth  of  . 

Pterosauria     .... 

Pulp,  the     .... 
„        degeneration  of 
„        nerves  of 
„        vessels  of  . 

Python,  teeth  of  . 


139 
231 
109 
31.5 
198 
137 

328 
426 

234 
227 
247 
233 
388 
401 
325 
,225 
215 
428 
76 
156 
248 
252 
254 
256 
47 
12 
280 
403 
222 
349 

363 

385 
382 
230 
259 

104 
107 
105 
ib. 
246 


Rachiodon,  teeth  of  .  .  .  247 
Rattlesnake,  teeth  of  .  .  .  248 
Rhinoceros,  teeth  of  .  .  .  316 
Rhyncocephalus,  teetli  of  .  .  243 
Rhytina,  teeth  of  .  .  .347 
Ridge  fonnul-.e,  of  proboscidea  .  362 
Rodentia,  teeth  of  .  .  .  364 
„  milk  teeth  of  .  .  .  366 
.,  enamel  of  .  .  45,  371 
Root  membrane  .  .  .  .  109 
Rostral  teeth  of  s^w  tish  .  .  222 
Rudimentary  teeth,  examples  of 

267,  271,277,311,347,382,430 
Rudimentary     milk     teeth.    e.\- 

amples  of        .       301,  399,  402,  423 
Ruminants,  teeth  of    .         .         .  334 
„  absence  of  upper  in- 


Salivary  glands  .  .  .  .37 
Salmon,  se.xual  weapons  of  .  .  236 
Sargus,  enamel  of  .  .  .  55 
„  teeth  of  .  .  .  .  235 
Saurians,  teeth  of  .  .  .  241 
Saw-fish,  teeth  of  .  .  .  .  222 
Scalpriform  incisoi-s  of  rodents  .  365 
Scarus,  teeth  of  .  .  .  .231 
Schreger,  lines  of  .  .  .  .  60 
Seals,  teetli  of  ...  .  388 
Second  dentition  .  .  .  .  194 
Secondary  dentine       .         .         .92 

Selenodoiit 338 

Serres,  glands  of .  .  .  .108 
Sexual  weapons,  teeth  used  as  .  273 
Sexual    differences    in    teeth    of 

Babirussa  330 
,,  ,,  in    teeth    of 

boar  .  325 

„  „  in    teeth    of 

deer  .  274 

,,  „  in    teeth    of 

dugong    .  345 
„  „  in    teeth    of 

elephant    .  350 
„  .,  in    teeth    of 

horse        .  320 
„  „  in    teeth    of 

monkeys 

409,  413 
„  „  in    teeth    of 

narwal     .  309 


INDEX. 


P\GE 

Sharks,  development  of  teeth  of  .  117 

„       teeth  of       .         .         .     .  21.5 

Sharper,  fibres  of         .         .         .  98 

Sheep's"  head  tish,  teeth  of       .     .  23-5 

Shrews,  teeth  of  .         .         .         .  396 

Simiina,  teeth  of     .         .         .     .  407 

Sirenia,  teeth  of           ...  344 

Sloths,  teeth  of        .         .         .     .  306 

Snakes,  development  of  teeth  of  .  12-5 

„       uon-venomous,  teeth  of  .  244 

„       colubrinc,  poisonous         .  217 

„       viperme,  poisonous     .     .218 

Socketed  teeth     .        .        .        .212 

Sphenodon,  teeth  of         .         .     .  243 

Sphyraena,  teeth  of      .         .         .  235 

Stellate  retifulum  of  enauael  organ 

131,  135,  156 
Stratum  intermedium  of  enamel 

organ           .         .         .  135 

Stratum  Malpighi,  the     .         .     .  108 

Stria5  of  enamel  prisms         .         .  50 

„     of  Retzius      .         .         .     .  51 

Succession  of  teeth  in  armadillo  .  306 

,,                „      in  lizards        .  239 

„                „      in  mammals    .  297 

,,                ,,      in  marsupials .  421 

„                „      in  osseous  fish  121 

,,                ,,      in  proboscidea  354 

„                ,,      in  reptiles       .  122 

,,                ,,      in  sharks        .  115 

,,                „      in  snakes        .  125 

,,  „      in     poisonous 

snakes        .  253 

Supernumerary  teeth  in  dogs       .  380 

Sus   babirussai  teeth  of  .         .     .  330 
Sus  scrofa,  teeth  of      .         .         .325 


Tapir,  teeth  of  .  .  .  .  316 
Tarsiijes,  teeth  of  .  .  .  432 
Teething  .         .         .         .     .  182 

Teeth,  equivalent  to  dermal  spines  2 
Teleostei,  teeth  of  .  .  .  224 
„  development  of  teeth  of  120 
Temporal  muscle,  action  of .  .  32 
Temporary  teeth,  eruption  of .  .  193 
Tetrodon,"teeth  of  .  .  .  231 
Theriodonts,  teeth  of  .  .  .  259 
Thvlacinus,  teeth  of  .  .  .  422 
'Ihylacoleo,  teeth  of         ...  428 


PAOE 

Tillodonts,  teeth  of  .  .  .  342 
Tillctlicrium,  teeth  of  .  .  .  343 
Tallies'  til.rils  .  .  .  .  64,  68 
Tomes' i)roresscs  of  enamel  cells  .  153 


j-ooiii,  ueuiiiiion  oi 
Tooth  sac 

142 

Tooth  germ 

113 

Tortoises,  teeth  of  . 

241 

Toxodon,  teeth  of 

339 

Trichechus,  teeth  of 

391 

Tusks  of  wild  boar 

325 

„      of  elephant,  foreign 

iodic. 

in       . 

352 

Typical  tooth 

• 

27S 

U. 

Ungulata,  teeth  of . 

314 

,,         molar  patterns 

jf 

319 

Yampire,  teeth  of 

.  401 

Varan  us,  teeth  of     . 

.     .  242 

„         dentine  of      . 

.    76 

Vaso-dentiue 

.       82  et  seq. 

Viper,  teeth  of     . 

.        .  248 

succession  of  teeth  in 
Viverrida?,  teeth  of 


Walrus,  teeth  of      . 

.     . 

391 

Wart-hog,  teeth  of       . 

328 

AVhaleboue       . 

311 

,312 

AVhale,  rudimentary  teeth  of 

311 

Wisdom  teeth 

22 

,,            of  lower  races 

of 

man 

416 

„            of  monkeys  . 

410 

415 

Wolf-fish,  teeth  of  .       " . 

229 

Wombat,  teeth  of       . 

432 

„        enamel  of 

53 

Wrasse,  teeth  of . 

235 

Ziphoid  cetacca,  teeth  of 


BRADBOnV, 


iNEW,    <fe    CO.,    PUINTERS,    WHITEFmARS. 


\ 


)«..;<*: 


^Z^ 


